You should be able to program this by going into System Preferences > Mission Control > and selecting the F4 key next to Mission Control.

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The only caveat is that you will need to hold down Fn while pressing the F4 key in order for it to override the shortcut that is already programmed on that key. ​You can also select a different key from that dropdown list if you prefer that. 

The hotkey functions for the F3 and F4 keys cannot be reprogrammed due to our keyboard's compatibility with Apple iOS devices which uses the Spotlight Search (F3) and the App Switcher (F4) functions.