Clutch slips in every gear but 4th?

We just recently put a spec stage disc in my buddys 97 z. The PP and flywheel did have a few hotspots that we hit with some sand paper to try and scuff up. The disc os almost brand new.

Anyways, it does slip now again, but not in 4th gear, he can put it in 4th from a 10mph roll and it will not slip. Every other gear will slip with like 3/4 throttle:confused:

Why is 4th some special?

below its torque curve. did you lay a straight edge across pp and flywheel to look for flatness, when they show heat stress they like to cup

Ya, its pretty obvious in 4th your bogging the crap out of your motor and getting ot power at 10mph thus its not gona slip.

Yeah, i understand that, but why would it slip then when i put it in 5th or 6th, that is more bogged down than 4th i would imagine.

Either way, the clutch is coming out, i was just curious.