hi One of my site has 2400 links from related sites with PR3 or PR4 in main page of sites but my site PR is now 3!!! Is it normal? I SEO this site completely.
It is normal, and for sure your website do not have 2400 links from PR3-PR4, because your anchor text appear on low rank pages.
Are you asking whether a PR 3 is too low considering that you have about 2400 links ? Google's toolbar page rank is based on a particular snapshot. Depending on your backlinks at that time, a Page Rank was assigned to you.
There are a lot of sites with PR4 to 5 but have only around 3k backlinks... it's not the quantity, it's the quality of backlinks you have...
2400 links for the PR3? I submitted my weblog to 10-15 blog RSS directories, and the weblog got PR 3 from PR 0 within 3 months.
try getting some PR6 - PR7 links & in the next update you will land up on a PR5 & again it depends on the quality and the relevancy of the backlinks to your site.