Maybe a tough call. Do you think the ring was already broken? What's the
engine mileage?

Maybe not a tough call. If you are doing it with the engine out, the heads
are already off, (doing both sides?) and you will be set up with tools to do
one piston, its not that much more work to do all four.

I've never done heads with the engine still in, so if that is the case
others might want to comment on how hard it is to replace rings with the
engine still installed.

I think in any case, if both heads are off,  you'll be set upup to do one
pistin so you night as well do the other three.

My two cents fwiw. Good luck.
Thomas Forhan