From what i know, for OEM ecu, evo 4 can use evo 5's ecu (plug & play) juz need to change the injetors. However if wanna use evo 6's ecu with evo 4, then need to modify 1 pin out (can't be sure which). These are for OEM ecu, but for Pwr FC i'm not sure. But logically thinking i think the Pwr fc should be possible to swap amongst E4,E5,E6. Anyways, got any good deals on a used evo 4 pwr FC cause i'm looking for 1 too :-)