@AnnaBonany12 First, I'd suggest making two separate folders on your external drive so you're keeping the game's program files and the user data separate. For example, you could create a Games folder and a Data folder, or whatever you want; the point is to direct the two sets of data to different locations.
If you installed Sims 4 through Origin, or you installed it through the EA App but can still use Origin, you can use Origin's built-in Move Game function to move the game's program files to the location of your choice. If you don't still have Origin installed, you can download it from here:
https://answers.ea.com/t5/Origin-Client-Web-Technical/Downloading-and-installing-Origin-Windows-and-Mac/m-p/11915666
Make sure the EA App and its EABackgroundService aren't running in the Task Manager before trying to open Origin. Then open your Origin game library, right-click on the Sims 4 icon, select Move Game, and choose the folder where you want the game to go.
If you installed Sims 4 through the EA App and can't run Origin anymore, which some people can't, you'll need to uninstall the game and reinstall it in the location you've chosen.
Steam has a Move Game function, called "Move install folder," under Properties > Local files. Right-click the game tile in your Steam library to see this function.
For the user folder, you'll need to create a symbolic link, as described here:
https://crinrict.com/blog/2020/02/moving-windows-documents-folder-to-external-drive-via-symbolic-link.html
However, please use the exact file path to the location where you're creating the symlink rather than writing %userprofile% in the command. If you're not sure what the full file path is, launch the game and find the file path here:
https://sims4.crinrict.com/eng/2018/07/how-to-find-your-user-folder/
Additionally, if you play in Spanish and are redirecting the Sims 4 user folder itself, rather than the Electronic Arts folder, you'll need to use "Los Sims 4" rather than "The Sims 4." (The only other affected languages are German, French, and Dutch.) If you like to switch languages, I'd suggest moving the entire Electronic Arts folder, since that has the same name no matter what language you're currently playing in.
If this doesn't work for you, please post the full file path of the Sims 4 folder, as seen in-game, as well as the location where you want the user folder to be on your external drive. Please also copy and paste the command you used, or post a screenshot of the command prompt window with the command showing.