Okay, so you have multiple PR sites. For example, let's say you have a PR3 site, a PR2 site, another PR3 site, and a PR1 site. If you put their links on each other, would it lower the PageRank? Or it increase? So for instance, your PR3 site has it's link on the other PR3 site, the PR2 site, and the PR1 site. And vice versa for each site. So what I'm trying to say is, these aren't "one-way" links.
according to google PR algorithm, in a prefect PR model, no matter how many links you placed on your page, has nothing to do with the page PR
The 4 sites have a fixed total amount of PR/ linkjuice amongst them - unless you add more incoming links from other sites than the 4 in the example. By linking all the sites together you'll simply spread the fixed amount of PR/ linkjuice around amongst them. So the PR of the PR 2 and PR 1 homepages might increase but the PR of the PR3 site will decrease - or rather the PR 3 homepages will have less PR to pass to the inner pages on their sites since they are passing it on to the PR 2 and PR 1 sites.
My bet is that you'll see little benefit. Probably none in your SERPs as link exchanges provide virtually no value. If you hosted all on different servers with different sets of IPs, then linked them in directions where they're not reciprocating (link each to ones that don't link back to it), then you might get some benefit. However, I'd spend your time focusing on getting good inbound links from other sites.
I won't do this, it's risk the whole websites system when one site get google punishment. I totally get hurt by this before.
Do you mean cross-linking? It's same as link-exchange. Try to avoid it if they're not relevant to each other.
The only way I could see it hurting you is if there are indicators that these are all your sites and thus not natural links. For example if they are all hosted on the same hosting acct. This is usually a problem when someone hosts a bunch of websites on one account and then points them all back at each other trying to quickly inflate the link totals to each. But again, my gut reaction is this should not hurt, but I doubt it will help a whole lot either. Probably a small boost at best as the juice passes down hill.
Does those site hosted and have different IP's? Are they relevant enough? I think with that scheme it will be just fine but will not affect much the ranking and PR of each other. What will affects them is when one or more site gets bad things (bad backlinks, penalty or something related), everyone will be probably affected too. The same as they get good things too.