If the two marks from your piston-stop exercise are both exactly the same distance from the "0" mark on the timing tab, your existing marks are OK. Advancing the cam 4* shouldn't have any significant effect on ignition timing - will just move the torque curve down a little.
I don't recall how the Unilite distributors manage centrifugal advance - haven't had one for many years. I run my '69Z with 8*-10* initial advance, 26* in the distributor, all in by 2800 (for total timing of 34*-36*), and a NAPA/Echlin #VC-1810 vacuum advance unit that adds 15*, and is fully deployed at 8" Hg. and connected to full manifold vacuum.