In Googles eyes PR4 is higher. THe highest you can get is PR10. However higher PR does not always mean higher in the searches. There is a lot more than just PR involved in that.
frist obey these rules is good for start No Frames No Tiny text No Invisible text Keywords in Title Tag Keywords in Description Revisit Tag Configure Robots.txt No Text Graphics Move scripts off page Alt Image Tags H1 tags Anchor Text Keywords Content Keywords Keywords in file name No Broken Links Theme Site Map Keyword Density Competitive Review Custom 404 pages Word Count Page Size Keyword Popularity Bold Text Alt Image Tags Text Links Unique Title for Every Page Keyword Prominence Relevance Meta Keywords in content too No Cloaking Keyword Frequency Keyword Stemming Review Information Pages Guide Pages Review Pages Optimize PDF Submit to Directories Increase Link Popularity Don't Duplicate Don't Link Farm Don't Stuff Hidden Input
This is not actually a predictive tool, rather it shows what the current PR is e.g. the PR of the most recent update across datacentres.
I never reached 6 but I got a 5 on pbkill.com after 1.5 years of adding just tutorials, all the traffic was from search and other tutorial sites. I kind of wish I didnt sell it now, its like a part of me is missing lol.