a) Can the hacker steal all the coins in the wallet without access to the 4th seed and without any knowledge of which addresses belong to the wallet?
No, The hacker needs the master public key of the 4th seed phrase.
b) If the hacker knows some of the addresses associated with the wallet that hold Bitcoin, would they be able to steal coins from those specific addresses?
If the hacker knows an address and there's an outgoing transaction from that address, then the hacker can steal the fund.
If there is no outgoing transaction from the address, the hacker will not be able to steal the fund, even if the hacker knows the address.
Take note for spending fund a from a m of n multi-signature address, you need m out of n private keys and also the public keys assosiated with other private keys.