wow that is rare to have a fouth axis on that machine. I know it was an option but not many people bought it.
I would say your best bet is to trace the power wiring and encoder wiring and figure the pinout for the connectors. Just be very careful because the heidenhain controller and drives are very expensive to fix. If something ever happens to that control and you are looking at over 2K to fix it i would recommend a ajax/centroid. I have done three retrofits on my machines and i love them.
To bad you didnt get a series 2 interact 4 instead. a night and day difference from a series one. alot more rigid.