From what i can tell the whole tail light piece on the 4 door is larger because of the indicator, take a look at the back of a 4 door and a 2 door, there seems to be quite a difference in size, i had a quick search but didnt come up with a lot, but it was only quick, so mayb have another look around the forums, though i do still think that the 2 door lights are smaller
check out this thread " http://www.skylinesaustralia.com/forums/t1...p;hl=R34+lights "
all pictures of R34s, you should be able to see the difference in there, some nice pics.