You are long past any upgrade eligibility cycles for CS4 and even if there were, you would only be able to upgrade to current versions.
Go find an old version on eBay. That's one of the very few ways you can get it.
I Have brought CS5 recently but it is not able to upgrade to new version since my pc is 32 bit (x86)
Well, yeah -- AE CS 5 is a 64-bit application. You need a new computer and operating system. Sorry, no 32-bit versions! I thought you would have been aware of that.
And make sure you get at least 16 GB RAM in that new machine -- you'll need it.