4's,5's,and 6's,are the norm simply because these particular shot sizes carry enough down range energy to penetrate a gobblers vital areas via head/neck via brain/neck vertabrae.But a few factors play into this such as knowing how far is 'to far' to shoot.Most turkey hunters will tell ya including this one keep your shots inside 40 yards.But even with that said as SJ pointed out his own gun patterns 5's well,so its up to you to pattern your own turkey thumper to find out what particular load/shotsize your gun patterns consistent up to that 40 yard mark.
Also in many states It is illegal to turkey hunt with shotshells loaded with shot heavier than 2's,somake sure and check your regs.
My own gun likes lead 4's,the turkeys dont!!......