When shooting strictly lead or copper plated lead, I always preferred #5's over #6's and #4's.
Sure #4 lead would work best, Energy wise, but the patterns weren't that great!! #6's pattern best over #4 and #5's, but have that energy problem at the 40+ yard mark, they seem to run out of steam!!
So #5's are the perfect compromise, better patterns than #4's, and better energy levels than #6's.
Now having said that, with the advent of hevishot hitting the market several years ago, and seeing first hand the damage done to a gobblers head while using it, I'll never go back to lead or copper plated lead loads ever again.
I'm presently shooting Nitro H378 3" 4X5X7 plated tri-plex hevishot loads. I also see that Winchester and Federal have both jumped on the hevishot train as well. I did try Remington's earlier version of their Hevishot loads when they had an agreement with Environmetal to use their products years ago, but Remington has since decided to come out with their own product. Like mentioned before, now that hevishot has proven how superior it is over lead, I'll never switch back to lead.