Hmm...I can't find anything on the net about any additions to the GH version of GT4, but I can't comment from first-hand experience with both versions, sorry...
However, they usually trumpet it when they make additions to GH games, and in the case of something huge like GT4, I'm sure they'd let people know about it beforehand. I'd say you should be able to buy the non-GH version without fear of missing out on anything. If they did add anything, I'd be hugely suprised that they didn't make a big ballyhoo about it.
[quote name='Logical Dolphin']I disagree. Most GH titles are usually exactly the same as the regular editions.[/quote]
I think he meant that if there are any changes to the game itself at all when it becomes a GH, that those changes are almost always additions. And the only reason you have to say "almost always" is because of GTA:SA, from which they removed Hot Coffee, but they removed that from printings prior to it going GH, actually.