Can 5AR4 be replaced by 5U4G or 274B?

Its really not odd that tube X can be universally substituted for Y and not the other way around. Tube X may be be able to handle more voltage or current than Y so it can deal with whatever Y had to handle, but Y can't handle what X can. However, Y could be substituted for X in some circuits if that circuit doesn't exceed its limits. You would need to know more about the circuit and the tubes to determine that. There are other parameters such as inter-electrode capacitance, plate resistance, filament current, etc. that could keep a tube from working well in one circuit when it would substitute just fine in another.

I'm sorry I can't answer the original question as I'm on the road and my tube manuals are at home.

I looked up the specs for the 5AR4 and 5U4G. The 5AR4 can handle about 10% more current, and the 5U4G can handle a bit more reverse voltage, but neither is enough to really make a difference. The big difference is the 5U4G filament draws over 50% more than the A5R4. That means a 5U4G could draw more than a 5AR4 circuit was designed to handle, but not vice-versa.

I couldn't find anything on the 274B.

The 5AR4 has a controlled warm-up characteristic, but otherwise handles conditions similar to the 5U4. The voltage drops are a little different too.

Generally if a 5AR4 is in an amplifier you can count on it being there either due to space considerations or the designer is being sensitive to the damage that will result to the power tubes if the B+ appears immediately, which is what will happen if a 5U4 or similar is used.

So don't try to sub it if you like your power tubes.