PR isn't a site wide thing. PR is based on the backlinks to a page, so it's possible for all your pages to have different PR and the main page doesnt have to be the highest.
Most sites you find with the highest PR for the main page is because that's the page that's promoted for. (ie. most of the backlinks are made to domain.com). It doesn't have to be that way though. If most of your promotion is to domain.com/page4 then that page will likely have the higher PR. Usually a lot of the PR will be transferred up to the main page through internal linking, but depending on how your internal linking is setup that isn't always the case.
It really doesn't matter though. Focus on ranking in the SERP for plenty of keywords and the PR will naturally improve over time as well anyway. PR is cool, but your SERP ranking is what's important. Unless you need a high PR because you want to sell your site or you want to sell adspace on your site, the only reason to really care about PR is for when you're looking for sites to get backlinks from. PR isn't going to get any traffic to your site, a good SERP ranking will.