For lead I recommend 5s as they offer a little longer range knockdown than 6s yet have a higher pellet count than 4s. But when shooting the new lead alternatives I have seen no reason for strictly 4s or 5s. I recommend that most of my customers who are buying HeviShot or Extended to try 6s. Being heavier than lead they have the knockdown of larger shot but still have a high pellet count for better pattern densities.

Now, I know I amplaying the same old tune I always play but since it was recommended in the original post... have you tried Nitro HeviShot and Rhino choke tubes? If you like less than a dozen or so pellets in the head/neck area at 40 yds you might like to know that most 3.5" guns will 230-270 pellets inside a 10" circle at 40yds with a Nitro triplex (4x5x7 HeviShot). My 870 Supermag shoots the Nitro very well through a .665 TruGlo (the old 3bands of ports, not the cheapies of today) but even it pales too my 835 shooting the Nitro's that are made for 835/935 Mossbergs. With a .695 Rhino and 2 1/4oz 4x5x7 Nitro my gun put 22 pellets in a 12 oz drink can at 60yds!!! (Yes 60)

The secret too lead alternative is NOT to overchoke it. To many guys think they must choke it like lead, yet it patterns like steel. Most 3.5" guns will shoot lead alternatives better through quality .660-.670s.
RA