I can't play The Sims 4

Otherson​  You can simply transfer your saves to the new Sims 4 folder.  They go inside the saves folder, naturally.  But if you have mods or custom content you want to use, I would test those files first, to see what (if anything) makes the game stop working.  If you have a lot of files to go through, the 50/50 method can make the process much more efficient:

https://modthesims.info/wiki.php?title=Game_Help:Finding_Problem_Custom_Content#The_50.25_Method

Aside from saves and mods/cc, you can transfer the contents of Tray, which contains saved households and builds, plus Screenshots and Recorded Videos if you care about those.  The rest can be discarded.