My personal impression of Part IV:
This one completes the "prologue" part of the series. With this film, the entire setup of Knk was explained, Shiki's dual personality, her relationship to Mikiya, how she got the Mystic Eyes of Death Perception etc. Now the "real" story will begin and the three remaining movies will all be roughly two hours in length, which is a good thing.
Maybe it comes from by fanboyism towards TYPE-MOON works, but I can't find any other word best fit for describing my personal thoughts about Kara no Kyoukai than "genius". Despite the fact that after four movies the actual story like hasn't even really started yet (since the first four were pretty much meant to build up the stage and setting), the movies have never even be remotely boring or felt too stretched. I really enjoy the way of storytelling as well as the achronological order of the first four movies. The atmosphere and mood of the series is something I haven't experienced so far. This is something entirely different from, let's say, CODE GEASS. Similiarly, you watch the one episode/movie and at the end of it, you want more. Yet, while CG pretty much ends every episode with a dramatic cliffhanger, you don't have such a feeling with KnK. It builds up the atmosphere and experience in a different, deeper, more complex way.
Meh...too lazy to come up with fancy sounding explanations right now.
In short: Best [tornado fang]ing Anime of the entire year, more people should watch it. If people say NGE is a masterpiece, in a serious and honest way, I say Kara no Kyoukai is.