If the rod is graphite, and 4 oz. is not the line weight itself (used to be based on the weight of 30' of line I think) it sounds like it is about a 6wt. How old is the rod? I have had plenty of good time with a rod that has line 1 or 2 weights over the rating, but trying to cast with a line under the rating can be very unproductive (for me anyway).

Good luck!!