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The 4s will be easier to pick out of the meat than 6s.
#4 steel will kill them just fine, but its more of a pheasant/ light duck load. You would be much better off saving those shells for a nontoxic area and just spending another $5.00 for some #7.5 LEAD dove and quail loads, since it really doesn't take very much at all to kill a grouse - Unless you really have your heart set on hunting grouse on pheasant release site.Another thing to keep in mind is the smaller the shot size the more pellets you have in your pattern, when hunting a hard to hit bird in heavy cover (grouse) where you might be shooting through leaves, small lead shot like #7.5 in an open choke is a decent advantage over steel #4's that usually hold a tighter pattern than lead at closer ranges. And in the early season most of your shots will be very close.