Which Mac do you have? Knowing will help decide on the the right USC connection for the drive. Instructions on what to provide here - http://answers.ea.com/t5/The-Sims-Mac/READ-FIRST-Sims-4-Mac-Help-Guides-and-Troubleshooting/m-p/4612396/highlight/true#M162
You can easily install the game on an external but the Sims 4 folder which contains your saves/mods etc. is a little more tricky as the game will only look for the data in your Documents folder in your home user account on your Mac. You can change this but it's not ideal.
