I have a Python script up and running that checks the signature with the same witness script (4 out of 6) using the cryptotools python package. I replace the first signature each time with one of the two additional keys and verify the script.
https://github.com/mcdallas/cryptotoolsUnfortunately I heard from others who tried it they get a lot of errors when installing the library. I will contact the owner of the package to try to solve these issues.
The solution is not perfect yet, since it does check the 6 signatures, but in theory, one of the six MultiSig holders could have generate the additional signatures making sure he/she is the first one to sign, the script would replace her own signature with the additional signature he/she provided.....
While writing this I think I found the solution!
Currently, I only replace the first signature each time with one of the 2 additional keys provided.
If I try to replace every one of the 4 signatures with each of the additional 2 signatures, this proves it is not a signature from one of the 4 key holders! If one of the key-holders from the original 4 signatures in the online transaction would have created the additional signatures, the script would fail when replacing 3 out of the 4 signatures present in the witness script, since the transaction would only be valid when replacing your own signature, in the other cases, 2 signatures are presented for one public-key leaving only two additional signatures, meaning only 3/4 signatures are valid.