Not sure where to ask, so trying here.
Is it possible to change the four characters that appear at the warband campfire?
I’ve bought a Warband bank tab and used it to transfer some gear to another alt on a different server that has not been linked to enable sending items or gold using the game mail system. Can game gold be put in the warband tab (I see a currency place) so other characters can access?
Yes.
The first four characters on your list will appear wround the campfire.
You can drag and drop characters up and down the list.
OR you can single-click a character, and arrows appear to the right. Then use the up and down arrows to move that character in the list.
Your second question is one for Blizzard, not us! 
I’ve been able to deposit gold into the warband bank from multiple characters, and have made one withdrawal so far. No problem with any of them but one of my addons that tracks money did throw a lua error on the withdrawal.
I’m sorry. After Tureja’s reply (and a night’s sleep!) I see I misread your second question.
Like Tureja, I have deposited and withdrawn money from my Warband Bank tab without problems. This will allow you, among other things, to buy items on different realms’ Auction Houses - which is nice.