Is there any diference between if I submit my site to a PR 4 category of a PR 7 homepage and a PR 4 category of a PR 4 homepage? I need your opinion on this
I would go with that solution too. A pr7 web site has more power and credibility from its pages rather than a PR4 web site.
PR4 in domain but PR7 in sub is rather strange, seemingly possible, but probably there is no such directories?
i think the pr of the page where your link is on is whats important, not the homepage. Of course both is better
Concerning passed PR there can't be difference because it depends on the PR of the page where your link is and the number of links on that page. However concerning recognition PR7 one is better. webgaya.