On my 2012 iMac, F4 used to bring up my Dashboard. But that was when I was running High Sierra.
When I did a backdoor upgrade to Mojave, it doesn't do anything.
On my 2012 MacBook Pro, F4 brings up the Launchpad.
However, you can use the Keyboard Pane in System Preferences and choose the Shortcuts pane to enable/disable F-key commands or create custom ones.