Sims 4 not loading on iMac due to 'already running' error

EA suggests resetting the game preferences: Open Finder, go to Documents > Electronic Arts > The Sims 4. Locate the Options.ini file and move it to the desktop. Then, try launching the game again.

If the problem persists, you'll probably need to contact EA support for further help.

Regards.

Try restarting the system and seeing if that clears it up.