I have just rebuild the top end with piper cam and I now have a misfire at 4000rpm any thoughts?

 

What did you do as part of the rebuild?
Is it new cam? Was it dialled in properly?
Messed with the ignition timing and/or set it correctly on rebuild?

Too many variables - some info of what you've done will narrow it down.

 

It's had a piper cam kit new can, lifters,springs and stem oil seals. Timing is spot on runs sweet until late I get 4g thenot it seems to running on 3 cylinders

 

Also put a new 4 branch manifold, new oil and air filters

 

"Timing is spot on" - assume you mean cam timing and it was dialled in then? (although using the "dot" would be close enough on a mild cam/standard skim head)
Did you run the cam in properly?
Have you checked and confirmed ignition timing since the rebuild?

Chances are its going to be one of two things:
-Ignition timing (12btdc on Premium (VPower) fuel)
-Cams wiped a lobe because... well Piper cam