This does work but I followed slightly different instructions:
Quick instructions:
1. Go to /system/library/extensions/ and copy AppleUSBmultitouch.ktext to somewhere else on your system as a backup. Now delete the original AppleUSBmultitouch.ktext in your extensions folder.
2. Download the file linked in the above post (it's a zip, but it will unzip to a modified AppleUSBmultitouch.ktext
3. Load up terminal (/Applications/Utilities/) and type in "sudo mv" (no quotes) then drag the modified AppleUSBmultitouch.ktext file to the terminal window - it should put the correct file location in there. Then drag the "extensions" folder to the Terminal window - it'll put the right folder location in there.
4. Press return, type in your administrator password and let terminal move the file over.
4A. Copy the Extensions folder to /system/library/extensions (you should have already deleted the original folder (after you backed it up))
5. Run Disk Utility and Repair Permissions.
6. Restart.
Enjoy four finger support! You can always install the latest combo update if something breaks.
Thanks to all who wrote the original instructions!