Brans Vademecum lists UU6 and V41 as equivalents. UU6 uses MO base — not pin-compatible.
Same heater voltage as UU5 (4V) — the UU4 is distinguished by its lower maximum anode voltage rating (350V vs 500V), not by heater differences.
Indirectly heated cathode — one side of the heater is internally connected to the cathode.
The UU5 can be used as a replacement for the UU4 (higher ratings are always safe downward), but the UU4 must not be used in UU5 circuits that exceed 350V RMS.
Pin Layout — B4
1Anode 1
2Anode 2
3Heater and Cathode
4Heater
Socket-Compatible Tubes ⚠ Not electrically compatible