It's not surprising that along with OS X Lion's release that Apple would also update its keyboards that highlight the software's new features. iSpazio posted a couple of images showing the new F3 and F4 buttons that toggle Mission Control and Launchpad respectively along with tweaked icons. These keys were originally earmarked for Exposé and Dashboard. If you have an older Apple keyboard, the F3 button automatically switches to Mission Control once Lion is installed. The F4 key retains the Dashboard toggle.
But, if you want your older keyboard's F4 key to access Launchpad without completely reverting all the F keys back to normal function keys -- the default in System Preferences -- it's doable with a couple of free workarounds.
The first is to download FunctionFlip, which allows you to turn individual F keys into normal function keys. Then use Quicksilver or another remapping software to remap the F4 key to activate Launchpad. Although the latest releases for both programs are for 10.6 (and show the older Exposé and Dashboard settings respectively), they work just fine in Lion. If you already have Quicksilver or you're interested in integrating it to your workflow, this is a good way to go.
The second is one commenter Jess came up with that takes Quicksilver out of the equation. Before installing FunctionFlip, go into System Preferences>Keyboard>Keyboard Shortcuts and select Launchpad and Dock. Make sure "Show Launchpad" is clicked, then click on the blank space to the right, which will allow you to set a keyboard shortcut. Press F4 (or Function key+F4) or whatever key you want to bind.
However, the F4 key defaults to Dashboard even after setting the shortcut, so you still need to install FunctionFlip. Once you've activated it and toggle the F4 key in FunctionFlip, the Launchpad shortcut will work as set.
Update: Post updated to add in second method.