Do rollers put more or less stress on your knee than riding on the road?

I am coming back from a pcl rupture, it's still overstretched and I need to build up the muscles to overcompensate, but after 4 miles on the rollers I have got a swollen and achey knee, would easy miles on the flat be better?

Dunno (my rollers remain unused 6 months after buying them).

On the turbo, the lack of positional change does hurt my left knee (which is a bit crooked form years spent playing hockey).

I think and easy ride on the road may be a good option to give you a feel for what you can do. Then try to translate that into a similar efort on the rollers.

maybe 4 miles was just too much to start with?

I'd ask a sports physio for a good recovery strategy. It might involve not riding the bike whilst building strength walking or swimming…