I am having trouble finding a s5 auto counterweight but I have lots of s4 one. Does anybody know if I can just lighten the s4 to make the same weight as the s5. If so what are the weights of each.

 

looking for the front or rear counterweight? I have the front available if you would like it.

 

It doesn't sound like a particularly good idea to modify the S4 counterweight. I would suggest that if it doesn't work out with gehzumteufel you can try:
1. Post a thread in the Buy/Sell section.
2. Call/email shops that work on RX-7s. They'll probably have something lying around.
3. Check threads where people are converting their auto S5 trannys to manual. PM them, they should have a spare counterweight and may not have thought about selling their counterweight.

 

you can't do this yourself, the only way to properly balance it would be to balance it on the Eshaft with the rotors and flywheel installed as a static piece.