Generally while reinstalling the game, nothing should be deleted. It's always recommended to make a backup though.
Your user files are stored here:
Documents > Electronic Arts > The Sims 4
You can copy the The Sims 4 folder to another directory to make a backup 🙂
If you start playing the game on another computer, the saves and library will not be on that by default. Everything you have uploaded to the community gallery will be there though!
You can also move the files to your new computer by uploading them on iCloud, putting them on an USB-drive etc. The folder you need are Saves (your saves), and Tray (your library). You can also move your Screenshots and other content if you like.
