I am a member of a forum that "upgraded" to v4. My advice is Don't Do It
It's very hard to customize, and the biggest problem I've seen is that searching for members' posts is impossible. If you click on profile and try to view all posts, you get a "No results returned," even if there are hundreds of posts. It also doesn't work by clicking a name and choosing view posts.
Multi quoting a post is non-intuitive.
Some have had troubles with navigation, because again, it's non-intuitive. Particularly for those trying to get new posts. Hard to find control panel without a couple of extra clicks, also.
If Invision Power Board is a better alternative, you may want to investigate it. But I personally recommend staying with v3 until there are no more security updates available.