A little update since I put a '05 STi steering wheel on my '00 RS, hopefully the '04 STi cruise control wires are the same. The '03 WRX steering wheel (replaced by STi) has the same cruise control wires as my '00 cruise control switch so this should work for both. Heres how to keep the STi cruise control switch, youll need to cut the plug off of your old cruise control switch so it'd plug up. There are 5 wires on the STi one since it has the "enable cruise mode button" on the switch. Heres the what wires you need to connect.
STi cruise control switch -> Old plug ---- Function
Yellow -> White ---- Set/Coast
White -> Yellow ---- Res/Acc
Green -> Green ---- Ground
These wires might have a stripe on them but I listed their main colors. Hope this helps.