Honestly, I think 3-4 hours is on the conservative side. While I wasn't there, I saw multiple images of signs touting a 300 minute wait for Frozen Ever After on opening day. While part of me can't believe that I'm typing this, it honestly wouldn't surprise me if we saw 6 hour waits for FoP on opening day, or within the first few weeks. The only caveat to that is if Disney does restrict entry into the land... but I don't see them doing that.
Regarding the single entry/exit point, there is a path that leads to Africa. It was restricted from inside during my preview window by three oh-so-excited cast members, which I'd assume was for logistical reasons. For the public opening, I'd imagine that they'll have both entrances open, as the entrance used for the preview would easily cause congestion not only towards the entrance to Pandora, but also to the pathways leading into the park.