[Arran]

The pinout is the same but the 5U4 has a 3 amp filament, verses 2 amps or less on the 5Z4. I think that the pinout of a 5Z4 lines up with a 5Y3, but the latter is directly heated verses an indirectly heated cathode. It's hard on the rectifier tube heater winding ,a d arguably the power transformer primary, but it's not going to detroy anything instantly like a dead short. I'm not sure why they came out with the 5Z4 other then it was one of the original metal tube types, come to think of it I don't remember running across a "G" style 5Z4 at all.
Regards
Arran