I ran the test you suggested and here's what I found out:

18F2550 with a 20MHz xtal, HS, postscaler (/2), DEFINE OSC 20
makes 1ms into 4ms, which is what works perfectly for my situation.

18F2550 with 20MHz xtal, HSPLL, any possible postscaler, any possible DEFINE OSC
can't make 1ms into 4ms.

I would need to use either a higher than possible DEFINE OSC statement or a larger postscaler option. For example, DEFINE OSC 64 with postscaler (/6) would work but 64 isn't compiling.
A postscaler which is (/8) could help too but it's not an option.

Seems I should just use a different xtal. I could get that same 1ms becomes 4ms situation if I use a 12MHz or 24Mhz xtal for this circuit. Are 12 and 24 equally usable? Maybe 12 would be better because less noise?