Why is 3-4 finger multitouch support not available (not even listed as trackpad preference options) on my late 2008 white plastic Mac book?
The trackpad in the 2008 white plastic MacBook isn't capable of supporting multitouch gestures. As it's a hardware limitation -- meaning the trackpad hardware can't sense input from 3-4 separate fingers at once -- there aren't any hacks to get around the limitation.
One way to get full multitouch support would be to get a Magic Trackpad. They'll work with any bluetooth-enabled Mac running Mac OS X Snow Leopard v10.6.4 or later.