Is it possible to make a multisig address with 4 keys (A, B, C, D) that requires 2 signatures to broadcast a transaction, but one of the signatures MUST be from A?
You mean the 2 signature must be from A? Meaning the A have 2 signature?
Yes it's possible but take note that this A can broadcast a transaction without the other cosigners but it defeats the purpose of multisig wallet.
Unless you talking about 1 signature all of the cosigner(parties A,B,C,D) have their own keys and must share the public key to A to create a multisig wallet(same goes to (B,C,D).
Take note all of them can sign a transaction but since you said you have 4 co-signer and 2 signature only requires 2 co-signer(2 parties can be A and B or C and D) to be able to sign and broadcast a transaction.