A few people have been struggling to install COD4x to older Mac machines.
This is what Chatgpt recomends
A 2012 Mac is actually one of the better Macs for running CoD4x because it uses an Intel processor and supports Boot Camp.
The main thing to know is that CoD4x does not have a native macOS client. The CoD4x developers specifically recommend running the Windows version through Boot Camp or a Wine-based solution such as CrossOver.
Best option: Boot Camp + Windows
If your 2012 Mac has enough storage:
Back up your Mac.
Open Boot Camp Assistant in macOS.
Install Windows 10.
Install Steam and download Call of Duty 4: Modern Warfare.
Install the CoD4x client patch following the CoD4x instructions.
Launch the game from Windows and join CoD4x servers.
This will give the best performance and compatibility.
Alternative: CrossOver
If you don't want a Windows partition:
Install CrossOver.
Create a Windows bottle.
Install Steam inside CrossOver.
Install CoD4.
Install the CoD4x patch.
The CoD4x team mentions CrossOver as a supported workaround, although performance and compatibility can vary.
Check your macOS version
If you're running Mojave (10.14) or older, the original Mac version of CoD4 can run.
If you're running Catalina (10.15) or newer, Apple removed 32-bit app support, so the old Mac version of CoD4 won't run natively. Boot Camp becomes the easiest route.