I have a question in my mind, so i wanted to ask here.

Suppose that if a domain has 4 PR, and a person do not renew it and get expired. will it's PR be finished.
what happen if another person register that domain after 7 days of it's expiration.

Thanks in advance.

AFAIK, the domain will lose most, if not all, of its pagerank.

Google has access to registry information just like everyone else. It also doesn't hurt that Google itself is an ICANN accredited registrar. So, they're as privy as possible to domain name status.

This may help: http://blog.v7n.com/2006/02/21/expired-domains/

I found a answer, i had posted this question in digitalpoint forums too.
Thanks Guys anyway

You technically lose the PR, but generally good PR means good backlinks, and most of those generally stay intact.

So what was the answer to your question?

Expired domain should retains PR if you add fresh content that fits the topic and continue on your link building process.

From my understanding google checks registrant information, and a change would clear the PR. Though remaining back links would still provide rank.

You loose everything. Also google will have the info needed to update your page rank to the correct rank.

I don't know the answer to this. My assumption is you'd lose everything. But in searching for relevant posts on other forums this morning, I found conflicting theories - both ways.

I actually looked into this extensively about a month ago when I had the option to obtain "thevpsguy.com" for $60. The previous owner Ian(now XenVDS.net) had let the domain expire, but in the last few years he'd generated a good amount of backlinks. At that point the domain had a PR of 3.

My original intention in purchasing the domain was to try to dispel "the vps guy" vs "vps guy", but then I considered I could take advantage of his existing back links and PR. Long story short I choose not to as I didn't want to confuse potential and existing customers with offers he'd listed on forums, and reviews of his service reflecting on me(although all seemed to be quite good). Within a week of the domain being parked the PR dropped to 2..

Actually PR is something that associates with the URL not the domain itself. For example compare the PRs of the following URLs

1. Where to get cheapest COM domain name

• http://www.hostingdiscussion.com/index.php

If you change the URL you might loose the PR. For example, if you have a PR of 5 for mysite.com/buy.html and if you change the file name of it to buystuff.html the resulting URL (mysite.com/buystuff.html) might have PR of 0. Because the number of inbound links to mysite.com/buystuff.html is zero.

the domain will lose, but since the backlinks will remain, i guess it will still have some rank, but if not renewed,it will become useless

Yes it's always best to prevent a domain for expiring.