I use #5s because they are what I took my first turkey with and I was very impressed with the way they swatted that bird.  That said I don't think you would be wrong to use #4 shot. 

In the latest Cabela's catalog I saw a duplex turkey load being advertised that was the #6 shot filling the spaces between and around big #2 pellets.  A straight load of #6 shot seems light to me.  I don't think the small pellets would deliver the range that a load of #4 or #5 pellets would.

If you can't make up your mind, I'll send you two or three of a variety of brands and you can try them all out before you commit to buying a bunch of them.  In my gun Winchesters seem to shoot better than Remingtons.  I don't know why.

I reload my own shells for my 10 gauge and I use #5 lead along with 2 1/2 ounces of shot.  I get really good patterns all the way out to almosst 60 yards, but most have been killed at 25-35.

I love turkey hunting because that allows me to get rid of some of my 10 gage lead shot that I have had for years since lead was banned for water fowl.  I still have a few boxes of BB shot left over from that and it works wonders on big gobblers at long range.  My only problem is that with only one or two shots a year with the lead shot it is going to last quite a while. 

I've never shot a turkey with a shotgun before, but my son has and I watched him wack a huge tom with a 3" mag #4 lead shot, it just dumped him in his tracks!

I've only shot two turkeys so far (NV not exactly the turkey capitol of the west) but both died right now with a tight pattern of 6's.  From all I found out researching it, 5 & 6 shot seem to be the most favored.  Your looking for dense patterns for multiple head/neck hits.  The first bird went down from a 12 gauge 2-3/4 ", 1 1/4 oz heavy pheasant load from my 870 at about 19 yards.  I did buy about 5 different turkey loads & found that the cheaper load I ended up using was consistently shooting tighter patterns out of my gun.  The second bird died at 39 yards from a 1.5 oz (approx.) load of heavy shot from my sideXside muzzleloader 12 gauge.  This was in a wildlife management area where lead shot wasn't allowed.

From what I've learned from research & from talking to serious (read that addicted) turkey hunters, you're looking for at least 8 - 10 neck/head hits out of a pattern preferrably out to around 40 yards.  I've never heard of anyone purposely aiming for a body shot unless they are throwing arrows.  Also it seems like 90% of the shots are at less than 40 yards and at those ranges 5 & 6 shot have more than enough energy for lethal hits with more pellets providing denser patterns.

I shoot 12 gauge #4 shot 3 1/2 inch ultra mag.  My dad had been turkey hunting for a decade before I gave it a try, and that was what he recommended.  It worked for me last year.