Have you ever read the SH3 Mods forum? There is a thread there devoted to just that - things taken from SH4 and applied to SH3.
http://www.subsim.com/radioroom/showthread.php?t=124207
As for SH4 itself, my machine won't run it, but most of the people playing it love it; and the new U-Boat add-on has everyone speculating about a complete upgrade, so u-boaters can have the awesome graphics and crew management as well.