I installed 10.5.6, no four finger swipe.
I tried manually adding the keys to the .kext file, rebooted, nothing.
However, when I used the .kext file downloaded from
http://macvn.com/forums/showthread.php?t=12460 - i have four finger swipe with no other modifications to 10.5.6 on my 17" early 2008 MBP.
Instructions:
Install 10.5.6
Download the kext file, and copy it to /System/Library/Extensions - I went to terminal and ran 'sudo mv /Users/<me>/Downloads/AppleUSBMultitouch.kext /System/Library/Extensions'
Run Disk Utility, select your main hard drive, and click the "Repair Permissions" button.
Reboot.
Four finger swipe immediately works, and shows up in the trackpad preference pane!