Radix-4 FFT versus Radix-2
It depends. Theoretically you can save a few multiplies with a radix-4 as radix-4 has a 1/4th the number of butterflies and 3 mpy + 8 adds per butterfly (if properly structured) and the radix 2 has 1 mpy + 2 adds per butterfly.

So in terms of multiplies it's a bit better, however there is higher complexity in terms of code structure, exception handling, coefficient management, register management, digit-reverse addressing, etc. 

So it's only an advantage if the number of mpy is the limiting factor which for most hardware these days is not the case.