Sorry to keep you waiting, but I've got both copies (bought the r1 first then won the r4) and wondered about the very same thing. The main problem I had with the region 1 version was that had poor contrast despite a very clean image. At first when I popped the r4 into my player it looked pretty identical, aside from the better PAL resolution. However, when I put the r1 back in it was obvious how over-bright the picture on the r1 was compared to the r4. The title card "Mr.Orange" before Tim Roth's scene in the diner is the best evidence of this - the black on the r4 version seems to be a lot more stable and overall the picture is noticeably better than the r1 edition. Another thing of note is that some of the extras have swapped discs. On the r4, the critic's commentaries are now on disc 1 with the movie and the deleted scenes and interviews have now moved to disc 2. This happens to be a good decision, as the critics commentaries on the r4 are now presented in widescreen (as opposed to this being fullscreen on the r1).
As far as I can tell the region 4 is identical to the r1 in the extras department, it DOES have DTS but I think the r4 is missing the DD2.0 track (no big loss)... I might also add that I the r4 cover looks a lot nicer!