sims 4 download on to USB

User data is still going to be saved to the user/documents folder on the system drive. To answer your question the game core can be installed to an alternate folder but the user data is hard coded to the system drive. You can create a symbolic link and move the user data but the game is still locked to the computer you installed it on. This will make finding the user data more difficult - you have to remember where you put it. While it is possible I don't recommend using a usb drive due to the data transfer rate limited to the speed of your usb drive which is probably slower then a storage device connected to the drive controller in the motherboard. Get a larger hard drive and image your smaller drive to the new larger drive.