Does NOT ignore the previous sequels
I watched this film and I know that they do not ignore the previous sequels 2 and 3. If it did do so then they shouldn't have named it Psycho IV. I know this because they mention the murders Norman committed "almost 30 years ago" which was the first one, and then Norman mentions a different inceident saying "The last time i killed was 4 years ago" and Psycho III was made 4 years before this film. So they're obviously following that time frame. Of course he's talking about the events of Psycho III.
Also, when that programmer lady for the radio show does research on the whereabouts of Norman Bates she says "The Bates Motel has since been closed down and Norman Bates has since moved away since the last string of murders." That's the key word. LAST. If this movie ignored the other sequels there would only be one string of murders and you wouldn't call it the last string of murders that occured. Even though, the paragraph above is proof enough, I just wanted to dwell on it.
Oh yea, these aren't the exact quotes but they're still on the ball. Psycho IV does not ignore 2 and 3. They are all one. However it does ignore Bates Motel, which is absolute rubbish in my opinion anyways.