How To Create Mods Subfolders in Sims 4 

You can create subfolders in your MODS folder in order to sort the custom content for easy management by editing the resource.cfg file.

Before you begin, make sure you make a backup copy of the resource.cfg file you find in your Mods folder and save it to your desktop (or some other place you can remember).

Next you open the resource.cfg file that is in the Mods folder in a text editor like Nodepad on Windows or TextEdit on Mac. On Mac, just right click on the file and select "open with" and then "TextEdit".

Then you copy/paste the following into the file, replacing the original content of the file, then save and close it:

Priority 500
PackedFile *.package
PackedFile */*.package
PackedFile */*/*.package
PackedFile */*/*/*.package
PackedFile */*/*/*/*.package
PackedFile */*/*/*/*/*.package

Important!! Make sure you don't accidentally rename the resource.cfg file. If it ends .cfg.txt. delete the .txt.

The amound of */* determines how many levels of sub folders you can make. Normally the game can only look 2 sub folders deep. With the above code you can have 5 levels of folders in the Mods folder.  That means you can organize your sims 4 custom content in folders within folders, such as a folder with [Clothes] which then goes 4 levels deep. 

If you forgot to make a copy of the resource.cfg file in your Mods folder, down worry too much. You can do a google search and download it again on various sites or simply edit the file again to the below (original) code:

Priority 500
PackedFile *.package
PackedFile */*.package