For some reason the game doesn't cloud save from steam to the steam deck so I figured I write a guide on how to transfer your save manually. I dont write guides so the formatting prolly sucks

First, find your THPS 3+4 save data in C:\Users\yourname\AppData\Local\IronGalaxy\THPS\Saved\SaveGames
You wanna copy all of the .SAV files in this folder.

Next you wanna transfer those files to the Steam Deck. (idrk how to do this easier but i uploaded them to discord then downloaded them on the deck lol)
You wanna then locate the save location for the game in Home\.local\share\Steam\steamapps\compatdata\2545710\pfx\drive_c\users\steamuser\AppData\Local\IronGalaxy\THPS\Saved\SaveGames

Paste all of your .SAV files in there. Once thats done, launch the game and you'll get to use your save. If only cloud saving worked lol