Vacuum advance modification might help with that. Does it do it at WOT?
When you said it does it at 4000 I assumed you meant at WOT. You must have some real deep gears to be turning 4000 rpm at 50-60 mph.
OH yeah, as far as the adjustment of the vacuum advance. The adjustment only affects when it comes in, not how many degrees of advance it produces. Dialing it to come in later can help in some instances but not all.