I'm not sure what you mean by asking what can you delete? Are you trying to delete some of your previously saved games that you no longer play?
It may be best to do this with a copy of the saves folder, not the original one. That way if something happens, you can get your files back.
1. Sort by Date Modified.
2. Assuming you've used Save As instead of save. -- If you have not played these saves in a long time, you can most likely delete anything before the date of say Feb.28th or the last time you played the game.
If you're trying to do a little spring cleaning and just want to get rid of the backup files the game makes. You can then delete the files that have .ver0, .ver1 - as these are backup files. Be careful not to delete the .save files as these are your main saves. See example below.
For example:
Slot_00000002.save (Main Save)
Slot_00000002.save.ver0 (Backup 0)
Slot_00000002.save.ver1 (Backup 1)
Slot_00000001.save is the auto save. If you've used Save As to save your game and given it a new name. You can then delete the auto save file.
The game does not use the auto save file any longer - Slot_00000001.save . It is safe to delete this file and it's backups. - .ver1, .ver2, etc.
Here's a link that may be of help to.
Restoring a Saved Game
http://www.carls-sims-4-guide.com/forum/index.php/topic,21519.0.html