I would give it a few miles, check it again before the warrenty ends and see where you stand. The compression could be a worn cam or lifter, valve adjustment, valve seat or valve face, bent valve, worn rings or cylinder, or even a cracked piston. You cold eliminate a bunch of these since its rebuilt, but one or more is letting the compression out or not letting enough air in to #4 cylinder.
Get some highway miles on it to really seat the rings and valves. Get everything adjusted the best you can and then retest. The worst that could happens is #4 needs a new guide and valve.
Joe
