Fosi Audio IM4 IEMs Review
A single dynamic driver IEM with open-back aluminum alloy shells, listed at $93.34 in Black or Silver. The specification section gives 21Ω, 109dB, 20Hz-20kHz and 7g per earbud, with a 3.5mm gold-plated plug.
Our verdict
Fits buyers who want a single dynamic driver IEM at $93.34 with stated impedance, sensitivity, frequency range and weight, and a 3.5mm cable on 2-pin connectors. Look elsewhere if you need a balanced plug or a reference level for the sensitivity. The main caveat is that every figure is the store's own, and the page calls 21Ω an output impedance in one place.
- Best for
- Buyers who want a single dynamic driver IEM with open-back shells and a stated 21Ω and 109dB.
- Skip if
- You need a balanced plug or a reference level for the sensitivity.
Why 3.5/5: Held back by contradictory terms on the store's own pages and by figures that no independent test was checked for in this review.
The page describes Fosi Audio as known mainly for portable and desktop audio gear and presents the IM4 as its first pair of in-ear monitors. It is a single dynamic driver model listed at $93.34, and the specification section gives 21Ω, 109dB and 20Hz-20kHz with 2-pin 0.78mm connectors and a 3.5mm gold-plated plug. The page describes a beryllium-plated PU suspension diaphragm, N52 dual magnets and open-back 6063 series aluminum alloy shells.
Specifications
| Spec | Page says |
|---|---|
| Impedance | 21Ω |
| Frequency Response Range | 20Hz-20kHz |
| Sensitivity | 109dB |
| Connector Type | 2-pin 0.78mm |
| Termination Plug | 3.5mm Gold-Plated Plug |
| Weight | 7g (per earbud) |
| Diaphragm | Beryllium-plated PU suspension |
| Magnets | N52 neodymium dual magnets |
| Housing | Dual-chamber, open-back, 6063 series aluminum alloy |
| Machining | 5-axis CNC |
| Cable | 5N OFC silver-plated |
| Nozzles | Two sets of swappable nozzles |
Options and pricing
IM4 Black and IM4 Silver are listed at $93.34 each with a crossed-out $99 beside it, and both were orderable when the page was read. The option field is labelled Model, but the two names differ only in the color word. The page lists one 3.5mm gold-plated plug, two sets of swappable nozzles, a 5N OFC silver-plated cable and nine pairs of ear tips, three each of Balanced Sound, Bass Sound and Deep Bass Sound.
How it compares
The FiiO FP3 14.5mm Planar Driver In-Ear Earphone is $89.99 and its spec line names a planar magnetic driver, while the Sivga Que UTG Flat Glass Diaphragm Dynamic Driver IEMs are $94.90 with a single dynamic driver per their spec line. The iBasso IT01X 1DD Entry-level In-Ear Earphone is $99.99 with one dynamic driver by name. At $93.34 the IM4 suits buyers who want 21Ω, 109dB and a 3.5mm plug stated on the page; the sibling summaries here do not give impedance or plugs, so check those pages.
Shipping, returns and warranty
HiFiGO sells from hifigo.com and says every order leaves its warehouse in China, usually 1-3 business days after it is placed, and says its delivery times are estimates, not promises. Its shipping policy gives the United States 1-2 weeks (about 10 days on average) by Priority and 6-10 days by Premium, both with duties and taxes prepaid by the store; Canada 2-3 weeks; the United Kingdom 1-2 weeks; and Australia 10-15 days. Express couriers (DHL, UPS, FedEx) take 3-7 days, but then the buyer pays any duties and taxes. No shipping page names New Zealand, PO boxes are not accepted, and the free-shipping thresholds differ between the store's own pages, so check the cart. One of its two shipping pages adds that a lost parcel on an order above $50 gets only a 50% refund. A US buyer reported a $40 tariff charge in August 2025, after which HiFiGO said it had added a customs notice at checkout.
Read the return terms before you order. The homepage advertises 7-day returns. Both of HiFiGO's policy pages offer a 7-day dead-on-arrival cover (a replacement or a full refund, with the return postage refunded) and also describe 30-day returns, but a request made 7 to 30 days after delivery loses 30% of the MSRP as a restocking fee (10% more if the packaging is incomplete), plus shipping both ways and duties, and the FAQ on the same pages says there is no refund or exchange once 7 days have passed. One version recommends reselling an opened product locally. Custom IEMs and custom cables cannot be returned, no return address is published, and buyers describe sending items back to China.
HiFiGO's warranty runs one year from the order date for parts and labor; its policy names desktop DACs and amps, IEMs and dongles at one year and gives cables and adapters three months. HiFiGO decides between repair and replacement, the buyer pays the shipping, and faulty items go back for service, not for a refund. Claims go to [email protected], the only contact the store lists: its pages name no company, street address or phone, and its terms are governed by the laws of China.
On Trustpilot, HiFiGO had 4.6 out of 5 from 378 reviews when we checked on 2026-10-28 (91% five-star, 5% one-star), and it had replied to about 75% of negative reviews; Trustpilot notes the store has not invited customers to review recently. The one-star reviews mostly concern the short return window and the deductions on returns, items listed in stock that turned out back-ordered, slow email support and a tariff charge on a US delivery in 2025. HiFiGO says its products are genuine and come in the brands' retail packaging; it does not claim to be an authorized dealer.
What we liked
- Specification section gives 21Ω, 109dB, 20Hz-20kHz and 7g per earbud
- Dual-magnetic, dual-cavity driver with N52 magnets and a beryllium-plated PU suspension diaphragm, per the page
- 2-pin 0.78mm connectors on a 5N OFC silver-plated cable, per the page
- Two sets of swappable nozzles and nine pairs of ear tips named on the page
- Open-back 6063 series aluminum alloy shells with 5-axis CNC machining, per the page
Worth noting
- Only a 3.5mm plug is listed; no balanced termination
- The description calls 21Ω an output impedance while the specification section lists it as impedance
- The page calls the two nozzle sets tuning nozzles but does not say how they differ
- The page text gives no reference level for the 109dB sensitivity
- The page text gives no isolation or sound leakage information for the open-back shells
Questions before you buy
How easy is the IM4 to drive?
The specification section gives 21Ω and 109dB, with no reference level for the sensitivity. The page says the dual-magnetic design is efficient but gives no power requirement.
Which plug and cable does it use?
A 3.5mm gold-plated plug and 2-pin 0.78mm connectors per the specification section, on a 5N OFC silver-plated cable.
What does open-back mean on this model?
The page lists an open-back shell design with grill vents on the face panel and ties it to its soundstage claim. It gives no isolation or leakage figure.
Where does HiFiGO ship from, and how long does it take?
From its warehouse in China, usually 1-3 business days after the order. The shipping policy gives 1-2 weeks to the US by Priority (6-10 days by Premium), both with duties and taxes prepaid, 2-3 weeks to Canada, 1-2 weeks to the UK and 10-15 days to Australia. Express couriers take 3-7 days, but the buyer pays any duties.
Can I send it back if I change my mind?
HiFiGO gives 7 days to report an item that arrives dead, damaged or wrong. Its policy also describes 30-day returns, but requests 7 to 30 days after delivery lose 30% of the MSRP plus shipping both ways and duties, and the FAQ on the same page says no refund or exchange after 7 days. Ask [email protected] before you open the box.
What warranty does HiFiGO give?
One year from the order date for parts and labor, for faults in build or materials (the policy names desktop DACs and amps, IEMs and dongles; cables and adapters get three months). HiFiGO chooses repair or replacement, the buyer pays the shipping, and a faulty item is serviced rather than refunded. Claims go to [email protected].
Sources & verification
What this review is based on and the day we last read it. Specs and claims from the maker are the maker’s own; the verdict and the rating are ours.
From the store
- Fosi Audio IM4 10mm Dual-Cavity Dynamic Driver IEMs product page: specs, options and price (the buy buttons go there)Specs, options and the listed priceRead October 28, 2026
- HiFiGO shipping policyShips from China in 1-3 business days; US 1-2 weeks by Priority or 6-10 days by Premium, duties prepaid; Express buyers pay dutiesRead October 28, 2026
- HiFiGO shipping and order tracking pageA second shipping text: Premium also for Canada and Australia, Canada free shipping from $99.99, lost-parcel rulesRead October 28, 2026
- HiFiGO refund policy7-day dead-on-arrival cover; 30-day returns minus a 30% restocking fee; its FAQ says no refund after 7 daysRead October 28, 2026
- HiFiGO warranty and refund policyWarranty by product class (1 year; cables and adapters 3 months); 30% fee for resealing an opened productRead October 28, 2026
- HiFiGO terms of serviceOperator named only as HiFiGo; governed by the laws of China; contact [email protected]Read October 28, 2026
Independent
- HiFiGO reviews on Trustpilot4.6 of 5 from 378 reviews (91% five-star, 5% one-star); the store has not invited reviews recentlyRead October 28, 2026
What we couldn’t verify
- The description calls 21Ω an output impedance while the specification section lists it as impedance; the page text gives no reference level for the 109dB sensitivity and no conditions for the 20Hz-20kHz range.
- Sound claims (soundstage, enchanting response, high-resolution wording, ultra-low THD+N) are the page's wording; the page text has no THD+N figure or graph, and the 10mm size appears only in the store title.
- HiFiGO's policy pages contradict each other: 30-day returns with a 30% restocking fee in one place, no refund or exchange after 7 days in the FAQ.
- HiFiGO's pages name no company, street address or phone number; the only contact given is [email protected].
Spotted an error or something out of date? Tell us and we’ll fix it and update the date.
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