It took me a while to get through book 4, Goblet of Fire, because that is absolutely my least favorite book. I might cause some controversy by saying that. I know a lot of people think that Chamber of Secrets is the worst, but I maintain... Goblet of Fire... too long... so many tasks! And I have a really hard time picturing Quidditch, so that crazy Quidditch World Cup at the beginning just completely looses me.
Of course there is a bunch of pivotal information in the fourth book. It is, after all, the central book of the series, so it would be expected that it is pretty important, and I confess that I cried quite a bit at the end, especially when Dumbledore makes his speech at the end of the year feast. That gets me every time, I swear. I think the most important stuff in the fourth book, though, is the information that we get about Lord Voldemort. Besides the apparition that we see of him as a teenager in book two, this is the first time that we actually see a glimpse of Voldemort as an actual human. We see his father's home if the first few chapters and at the end, we see him needing the assistance of his Death Eaters and Pettegrew, and we get parts of his life story. I admit, I think that Voldemort is one of the most interesting characters in the series--he is one complex dude. I actually think he might be my favorite after Dobby, on a purely academic level that is, don't get me wrong, he's totally evil, and as a Gryffindor myself I certainly do not approve, but you can't deny that he is interesting.