I would never invite randoms into any of my many Guild Banks... *shudder* I'd hate to let anyone see my alts logging, shuffling, gathering for extended periods of time. Such a bad suggestion. Then what, these randoms are just going to sit there and do what? Stare at the Gold reserves and Deposit tabs?
Simply put, create Guilds -as many as you need -with a few accounts you don't even need to ask for scrubs to sign your Charter for pennies, and you'll know full well that your Guilds are created in complete confidence and hassle free. With the WoD 7 Day trials you can even resuscitate expired accounts to aid you in this process.
That said, with Reagent Storage (and more importantly, changes to Crafting Reagent stack size), I have found that I now need less Guilds than ever. I have (checks spreadsheet) 7 Guilds for the purpose of Storage, however I'm now thinking of downsizing to just 3 with the changes to storage. The Gold Cap is still an issue, only storing a copper short of a Million gold per Guild but that's another issue.
For me:
MAIN -Bnet #1 Account X (Guild 1,2,3)
* Gold capped
GATHER -Bnet #2 Account Y (Guild 4, 5, 6, 7)
MAIN Guild 1 maintains 500k -enough for Raiding lifestyle. Crafting and shuffling is all done by hand and then sold on AH, with my wowuction API spreadsheet and a mix of TSM (which I loathe to use). Guilds 2 and 3 on MAIN hold a further 1 Million gold, I have yet to earn more than 3 million gold. I do not earn a single copper from COD (the stupidest suicide a bot can ever, ever make!) so I keep all of my Gold on MAIN as it is earned through Shuffling, Crafting, Raiding and AH -all by hand.
GATHER -Guilds 4~7 are my storage guilds and home to my Gather bots, filtering out mats to MAIN by Post, Trade or via a Friend who helps out occasionally (I will post him mainly Herbs though we exchange Gold, Materials, etc sometimes... it is not a one way process). So far it's worked out well, and I really don't know how important it is to have the Guild 4~7 on a seperate Bnet but it's an added feeling of safety (keeping Gatherer bots away from my MAIN and Gold) which increases my enjoyment. All in all I have 2 Guilds at Gold Cap (I'd never trust anyone to sell it to!) and my 500k Guild on MAIN.
I would recommend though (slightly off topic) that all Shuffling, Crafting and Selling is best done on MAIN by hand. I sell large amounts of Glyphs via the TSM addon, which is a pain, but at least I can genuinely say that MAIN is bot free but absolutely the money earner.
For you:
TLDR -as you have 4 accounts, create an 'AH / Gold / Shuffling / Crafting / Raiding' Guild on your MAIN (#1), keep some gold (not all) here. On one of your other accounts (#2), create enough storage (1 Guild or 10 -you decide!) for your Gathering. If you want to be super safe you can split your Gold Reserves across the other accounts (#3 and #4), but maybe you prefer to keep all of your Gold on (#1) your main, personally it's up to you -but as you seem conscientious I'd recommend you seperate your Gold from your Gatherer accounts.
(EDIT) you didn't specify whether you have a separate Battle net account -do you? I'd highly recommend a separate Battle net account if you are holding large amounts of Gold or Mains (characters, achieves). If all goes wrong on the Gatheres, at least you have your Main (on it's own Battle net) as a safety.