I lost the F4 binding too, and after some investigation I got it working again. It's a bit of a hack, but here's what worked:
- Install FunctionFlip, a free preference pane to modify the function keys.
- In FunctionFlip click F4: Launchpad, so it's checked.
- In System Preferences / Keyboard / Keyboard Shortcuts, doubleclick on the keybinding for Show Launchpad and press F4.
FunctionFlip should not be necessary: it should be enough to set the binding in Keyboard Shortcuts. The problem is by default the F4 key does not actually send the keycode for F4, it sends the magic code for LaunchPad. (Just like F8 isn't F8, it's pause iTunes.) And the Keyboard preferences pane won't let you bind that magic LaunchPad key. FunctionFlip solves the issue by making it so the F4 key really sends F4.
Note that FunctionFlip is a bit tricky. For it to work, you have to enable System Preferences / Universal Access / Enable access for assistive devices. Also there's a "Start FunctionFliip at Login" preference that you probably want to enable.
The FunctionFlip page points to two tutorials for LaunchPad: TUAW and Josh Sunshine's. Same procedure as I describe above.