@Junolie9 Are you trying to make your old saves accessible, or trying to play your old and new sims within the same save? If you just want to play your old saves, their files are located in the Saves folder inside Documents\Electronic Arts\The Sims 4. Save files can be transferred between computers, even between different operating systems; you just need to put them in the Saves folder on your new computer.
You may need to rename some of the saves you're moving, or some of your new ones. Each save is named Slot_0000000x.save, with x being a number or letter a-f. (The ones with .ver0 through .ver4 on the end are backups of the same saves.) Change the number to something else, so you don't have multiple saves with the same name, and they should all be available to load from the Main Menu.
If you want all your sims in the same saves, you'll need to save them to your in-game library and place them in the new saves. You may also need to cheat to restore their household funds, so make a note of how much they have, plus the cost of their house, before you save them.
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