You need protontricks and the numeric game id.

From a terminal, run

protontricks game_id winecfg

And it will pop up a config window for the proton instance for that game. From there, you can tell it to pretend to be windows 10 instead of win7. You should only need to change it once. (Well, maybe again if you change proton version, or maybe not. I'm not sure how steam manages its wineprefixes)

Appears it should work pretty well according to ProtonDB

https://www.protondb.com/app/12210?device=pc

You might need to install ProtonUp-QT and grab GE-Proton and set the game compatibility mode to “~~Proton Experimental~~ GE-Proton”

There are also suggestions of downgrading the game version to remove the launcher (Im assuming the launcher does the windows version check)

And this user’s problem/solution sounds identical to yours

I needed to switch to windows 10 support, while firtst launching it said that windows 7 and 8 was no longer supported.

To fix: in protontricks select GTA 4, Select default wineprefix, Run winecfg, A window should now pop up, at the bottom In Windows version select Windows 10, select Apply and OK Even if you see Windows 10 already selected reselect it and press OK.

Then it worked perfect.

Linux mint tends to use packages that are older but more stable than some distros. Have you tried using steam's flatpak which might be more up to date?