If you're having trouble launching Far Cry 4 on Linux with Steam/Proton, this procedure worked for me, but YMMV.

Hardware: Ryzen 5600X, Radeon 9060XT GPU, 16MB of RAM
Software: Fedora 42, running steam on Flatpak

1. Use Flatseal (or command line) to give Steam access to GPU acceleration (device=dri)
2. In the FC4 properties page:
- Make sure Steam overlay is turned OFF
- make sure you have '-skipintro' set in the parameters
- Set Proton version to 9.0.4
3. Using ProtonTricks or similar tool, manually install Ubisoft Connect into the Wine Prefix instance that this game uses. Set up Ubisoft Connect as you normally would and make sure it starts. Manually exit Ubisoft Connect when you authenticate.
4. Shut down your Steam instance entirely (maybe even reboot just to be sure all running Steam processes are terminated)
5. Try to start Far Cry 4 again. If you have done everything correctly, you should see Ubisoft Connect start. The game might not start automatically; in this case, you must right click on the Ubisoft Connect icon and manually start it.
6. Wait a good 30 seconds or so. I'm running the game on a fast NVME and it still took a while to start up. It should skip t he intro cinematic (which is what usually causes launches to fail) and you should now see the main title screen.
7. You may have some stability issues and some texture artifacts, but I found the game pretty playable at this point.,

Hope this helps!