This is vague and may be totally wrong, just my opinion. I've read in other places that RC difficulty is entirely subjective, but I find that hard to believe. The first or second passage to me is almost invariably going to be the easiest in the set. Every now and then I find a dual passage that appears third or fourth to be the easiest, but I find those easier in general, others may disagree.
I find that, in general, the first two are easier. If one of those is dense, it probably uses straightforward language. Furthermore, if the first or second passage is dense the questions will probably not require you to dig as deep into the passage or make complicated inferences like a later passage would. As for third or fourth, I usually find these the hardest. I took June and I found the third significantly more difficult than the fourth (to the point that I did not have enough time to adequately complete either passage and my test was deemed meaningless

) Once again, that can switch. Fourth could be way harder. Someone posted an important link in a thread titled something like 'Theory Behind Reading Comprehension' that I want to read before I start prepping for the September retake.