I would like to reassign my F4 button on my mac magic keyboard to something else. Unfortunately, when I try to reassign it, it opens Launch Pad.
Here's my keyboard Shortcuts panel… 
I'm able to remove the default action for other keys like the F3 key, but not for this one…
How do I remove this default action so I can setup a new use for that F4 key?
Best Answer
It's not that F4 is set as Launchpad, it's that you have your F-Keys set as 'special function' keys & that special function happens to be Launchpad. (The set of little squares printed on the key next to F4 - each of those icons represent the special functions)
If you look at the first tab of that pref pane - Keyboard - You'll see that box is unchecked…
Once you do that, your 'real' F-Key will come into play, with whatever you set it for in the Shortcuts pane.
Alternatively, you can temporarily swap special function/F-Keys by holding Fn