The 64 Audio Fourte is 10Ω at 114.0 dB/mW; the 64 Audio U4s is 11Ω at 107.0 dB/mW. This page is only about what they ask of an amplifier — not how they sound, which no specification on either sheet can tell you.
The U4s needs 7.0 dB more than the Fourte to reach the same loudness — 0.20 mW against 0.04 mW.
| Specification | Fourte | U4s |
|---|---|---|
| Impedance | 10 Ω | 11 Ω |
| Sensitivity | 114.0 dB/mW | 107.0 dB/mW |
| Sensitivity (voltage) | 134.0 dB/V | 126.6 dB/V |
| Power for 100 dB | 0.04 mW | 0.20 mW |
| Voltage for 100 dB | 20 mV | 47 mV |
| Current for 100 dB | 2.0 mA | 4.3 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.25 Ω | 1.38 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
23 sources stay below the audible-hiss threshold with the U4s but cross it with the Fourte (Kiwi Ears Allegro Mini, FiiO KA13, FiiO BTR15, 20 more).
280 of the 280 sources here reach 100 dB on both. That count is about power only — the output-impedance and hiss lines above are separate questions, and a source can pass this one and fail those.
Each model has a page of its own with the full source list, the hiss estimate and the provenance of every figure above: 64 Audio Fourte and 64 Audio U4s. Every figure on this page comes from what 64 Audio published, or is derived from it — what that maker publishes, counted.