I think most people here know that I am very "for" same-class groups, as far as maximizing realistic potential. IMO, part of this is because cast sequences are just a drain on DPS, it will net you a much higher DPS to manually cast spells than using castrandom or cast sequence. Doing this for even 2 separate DPS classes is nearly impossible, for 4 there is no way it could be done efficiently - you'd have to rely on castsequences or castrandoms for sure, which will lower your potential DPS.

Personally I think most people can agree that you should multibox whatever is the most fun. That being said I will say this: Regardless of the fact that having 4 different "synergistic" classes seems to have higher potential, realistically I doubt anyone will ever get as much real performance out of a group of separate classes than people are getting with 4 shamans or 4 druids, etc. Proving this is very difficult, but there are a lot of shaman or druid boxers out there, and a lot of mixed class boxers out there, and if you look at self performance people are quoting, I mostly see DPS for mixed class groups in the 1500-2200 range, and there are a lot of all shaman and all druid dps groups doing >4000 dps (or even if you remove bloodlust > 2900 dps).

Part of the reason for this is that, to get the numbers above with an all shaman group at least, you really have to do your casts manually. A castsequence or castrandom will just not net those numbers compared to manually hitting FS, manually hitting LVB, spamming LB for awhile, then manually LvB, manual FS, etc, repeat. Doing something like this for 2, 3, or 4 different classes would be impossible - for one there wouldn't even be enough buttons on a keyboard but even if there were you couldn't hit that many at a time in the right sequence without breaking something or messing up

So, I still say for min-maxing, 4 shamans or 4 druids is the way to go (I'm partial to shamans), but again, play whatever is fun.

For gear - for me thats been a non issue - there is only maybe 2 pieces of gear I have on my shamans that I got from instance drops, and getting 4 of each piece happened way faster than getting my 60 badges for my t7 gloves. Most "good" gear for multiboxers without raiding is either crafted or from badges, so heroic drops isn't really a big deal at all.

This is just my 2 cents of course.