If you are using the 20 gauge I would strongly advise you to use Hevi-shot and go with #6s and stay away from #4s. It is a small load to start with (1.25 oz in three inch shells) which means you want the most pellets in my opinion, therefore #6s. Keep your range reasonable (30-35 yards) and you'll be fine. My son's gun (Rem youth 870 express) patterned the Remington Hevi-shot 1.25 oz #6 great before I switched him to Nitros.

I posted a pic my son's first turkey taken Saturday with his 20 gauge. It has a Primos Tightwad choke and he was shooting the Nitro triplex load 4x5x7, 1and 7/16 oz. He absoluted smoked that turkey with multiple head and neck hits at approximately 28 yards.. Very impressive. Those hevishot #7s have plenty of energy at 30-35 yards and there are a zillion of them in there.I truly believe thatload is the ultimate for a 20 gauge.

Someday I may set up a 20 gauge to hunt with myself, but only if I can get my hands on the Nitro shells. That little 20 sure was a pleasure to carry around compared with my Mossberg 835 Ultimag.

Edit: One more thing - spend a few bucks and get a decent turkey choke, one that is ventilated to reduce recoil and strip the wad as the load exits the barrel. You'll get better patterns and your shoulder will thank you for it. You don't have to spend a fortune to get one that will work either.

Good luck

Mouthcaller