Allwyn, what if we don't end up using the fourth character? I understand that the game is ideally co-op'ed keeping in mind four characters. And how about the campaign playthrough? Is it like, all the characters would spawn at the same location and move to the same point say a dumpyard and finish off the opponents. What of the loot? Shared?
It's not mandatory to have all 4 players to start the game. I think Borderlands 2 has promised drop in and out co-op. So if you leave your slot open, anyone else can join in later on. Hence, if you are a player short, then three people can go ahead.
For the spawn part, I'm assuming it will be the same as the first game. If you all start the game simultaneously from a private room, all the characters would spawn at the last saved spawn point. Loot can be shared pretty easily. One player can drop his weapon, you can compare the stats and pick it up if you want to, else let go of it.