If funds were sent to A2+B but A4+D sign a transaction, how can they (A4+D) access the funds sent to A2+B (sorry if my newbie question seems a little dense) Wouldn't they be in two separate wallet addresses?
A n-of-m multisig creates one address that can be accessed by n people. I recommend to try it on testnet:
https://coinfaucet.eu/en/btc-testnet/get some testnet coins[/url], and play around with 3 wallets until you get a multisig address. Then fund it, and withdraw again. Doing so helps explain how it works.