The CF AN/PVS 502, when focused/"tuned" was/is every bit as good as the PVS4.
The pvs4 is considered gen2 as a result of it being able to shut down immediately, whereas the 502 is gen 1 as it needed to wind down.
The 25lp/mm was basically the same resolution for both. I spent a few nights with both units comparing side by side and found contrast etc very comparable.
The updated gen3 pvs4 is very nice
There are some components/parts that are shared between the 502 and pvs4.