Having said all that, it would be better if you installed TS4 on your laptop and moved other files to your external drive.
You should be able to choose where to save files when you start the download make sure that the drive's free space is at least twice the size of the download plus about 25 GB. If your laptop is short on space, you might have trouble downloading files even before you install them.
If Origin is already installed, you'll want to reset it with this mac-specific tool: There are a number of hidden Origin files you'd need to clean out when uninstalling. I'm not sure whether you're having trouble installing Origin or the game.
