Is there anything I need to be aware of when I use a 4/5/6 tranny in my 8/9 5 speed? Any issues with speed sensors or gear selector arms?

 

It works fine, I have done it to my Evo8 MR. Check the speedo sensor gear. It should be 28 tooth on 8/9 but it is probably 29-30 on those boxes.I have an Evo4 5-speed box but it was rebuild with new evo9 rs shafts and gears. I still have the 30 tooth speedo gear in the box and the speedometer reads too low. The 28 speedo sensor gear is MD747055.

And check the shifter cable ends at the box side. The bushings are probably 22mm both and seems like us model 5-speed boxes have different size bushings there (and the gear selector pins are different size).

 

 

Us model shifter cable bushing pin holes are about 8mm and 10mm and Evo4 5-speed gear selector pins are both 8mm and the cable bushings are 22mm OD and 8mm hole (both). So there is a mismatch. If you just could swap the gear selector housing from your old 5-speed box? And take the speedo gear from your old box too. And use your own 8/9 gearbox/engine mounts.