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/cryptotools

Unfortunately 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.

Small addition. After parsing the Partially Signed Bitcoin Transaction (PSBT) text shared for signing a 4 out of 6 MultiSig, I found that the first four who signed had their signatures ended up being in the witness data.
However, there is still a small chance it is not the order of signing but the order of key holders in the MultiSig that determine which signatures end up in the witness data which I cannot rule out.

Thank you all for your suggested solutions .
I agree that making two transactions would indeed offer the simplest solution, the Grin Community Council can use this method for the time being.
However, because I do think it would be great to have a general solution for MultiSig holders that does not have the costs of an extra transactions, I will finish a script I am working on to parse the signatures from the Partially Signed Bitcoin Transaction text (PSBT, BIP174) and I will provide simple Python script to verify all the signatures. This script will be made available as an open source solution.

For future references, I would also like to note that a transaction does not prove the time of signing unless a time proof such as a recent blockhash is provided in an OP_RETURN statement. So for perfect security, proof of time should be signed to, similar to how the text in the first Bitcoin transaction proofs its time of creation.

Link to PSBT, BIP174:
https://bitcointechweekly.com/categories/consensus

After some more searching I found this nice website that shows all WPA2 hashrates for all high-end GPU's using hashcat https://tutorials.technology/blog/08-Hashcat-GPU-benchmarking-table-Nvidia-and-amd.html
Apparently when you run Linux it is easier to setup the drivers for most Nvidia cards than for AMD, for Windows both works fine.
Find below the top performing GPU's and their hash rates using hashcat from the link above.

RTX 2080 Ti            758700 hash/s
Nvidia RTX 2080 Founders Edition   571400 hash/s
Nvidia GTX 1080Ti   576000 hash/s
Radeon VII 16GB   534000 hash/s
Nvidia GTX 1080   396800 hash/s
GeForce GTX 1070   285000 hash/s
Radeon RX 580   224000 hash/s
Radeon RX 480   185000 hash/s
Radeon R9 390X   200000 hash/s
Radeon R9 380X   145000 hash/s
Radeon R9 295 x2   347000 hash/s
Radeon R9 290X   163000 hash/s
Radeon R9 290   147000 hash/s


Unfortunately, no such overview exist for btcrecover (my favourite wallet recovery software).
The manual for btcrecover gives instructions on how to setup drivers for GPU support on Windows but not on Linux.
Although the algorithms are different it appears good mining GPU's also work well for wallet cracking. This excludes bitcoin Armory wallets which are GPU resistant

Sorry for not clearly stating my intentions. No, I will definitely not try to crack/hack other peoples wallets unless they ask me to do so and I am definitely not stupid enough to attempt to brute force find any privatekeys .
What I want to do is help other people recover their wallet password/passphrase after successfully recovering my own wallet recently using btrecovery. I know brute forcing is only feasible if the password is very short, so I  want to help people who have at least some idea of what their password might have looked liked, so using dictionaries or patterns to try possible passphrases.

In my case I did managed to find the passphrase purely using my CPU but it took some time and I realize in most cases some powerful GPU's are needed. My interests in bitcoin, cryptocurrencies and blockchain technologies have been sparked since I recovered my passphrase, and I would like to understand more about the core technologies as well as possibly become a service provider myself. I realize I still have to learn more before being able to help others to recover their wallet unless its a simple case of typos.

Thx nc50lc, this was the kind of information I was looking for. So yes it is more complicated than just running input passwords though SHA512 encryption. So I should check which mining GPU's are best supported by btrecovery and hashcat

Did I miss something... , is posting a link in a question somehow an issue?
The link to that article in my post is the number one hit on google when you search for “best gpu for bitcoin mining 2019”, so I doubt the maker of that article will need any links to their article to increase their ranking if that is what you are referring to. If there is another problem with my post, please explain.

Anyhow, I find it a bit disapointing that the first reaction to my first post on this Forum is "go in Hell" .

I am thinking on building a  rig for wallet password recovery/cracking using approximately 2000 euro/2200 dollar. My thoughs were that for this price range two AMD Radeon VII might be a good choise, if I can get my greedy hands on them . It those GPU's are note availble I select another one from this list of mining GPU's https://www.techradar.com/news/best-mining-gpu
Does anyone have a nice build for this price range?

Furthermoreo I have some questions/considerations since I am only just reading up on the technical details of mining and wallet cracking.
Does a mining rig and wallet cracking rig have the same requirements?
If I understand it correctly, mining is done by letting the GPU perform a (double) SHA256 operation with a certain shance/difficulty to mine a block. If I red it correctly, recovering a wallet password is done by running a lot of inputs (passwords) thourgh the SHA-256 protocol to generate a hash that matches the hash in a wallet. So basically the operation is the same, right?
Am I therefore right in this assumption that a good mining rig is therefore an equally good rig for cracking/recovery wallet passwords?