[392cobra]

RB,
With your engine flatlining at 4000 you are leaving alot of hp on the table somewhere.

If you hadn't already said that you have open sidepipes I would say you had the LSC stock sidepipes on.
This is what has shown up on the chassis dyno time and again with a heathly engine and restrictive exhaust.Right at 4000 rpm.

Before you go spending money on heads,cam,etc. I would figure out what is shutting you down.