I recently finished my "test" run with Cornish Xs and an 8x6 tractor. I butchered them at 7&1/2 weeks.
Live weight at 7 weeks for one of the cockerels was 6lbs 12 oz. (didn't weigh the pullets), which I thought was a bit low. So, yeah, a live weight of 4 lbs at 7 weeks sounds low to me.
At...