I have used 7½ shot and didn't have any problems and they fell dead to the ground too. I got a mess of #5 20 gauge shells for "pheasants", so I like those, and they work just fine. Prior to that I also used #6 shot. Maybe I am getting closer to the squirrels when I shoot, than some of the other folks on this topic, but never needed #4's, and only use the #5 as they were super cheap at the time I bought them. Tops of oak trees, no worries.
LD