Yep, it's also been established that the 4-plug plugs plug into 3-plug ECU, and there looks to be an equal amount of re-pinning to do (about half ish) So really, i reckon it would be foolish to go with 4-plug ECU, when you can get a 3-plug in there with the same amount of hassle. The one thing about the 3-plugs are you will need to get yellow injectors, and a compatible air flow meter.
Wiring wise, take a look at this PDF, and the northursalia link above
ECU Pinouts BC5/BG5a/WRX 4-plug.pdf
Also, the GTB 3 plug is the same as the impreza 3plugs, only it has 4 ignition wires not 2. Regardless, it may help to have both. i fount the little diagram at the top of the gtb one very handy
97-98 ECU pinout.pdf