Is it possible if i can change carrier with my iPhone 4 while im in contract with other carrier or not because the carrier i have now has a very very weak signal!
Whether you can escape your contract would be up to your carrier and the terms under which you purchased the iPhone and signed up with the carrier. If the carrier subsidized the purchase of the iPhone, you probably cannot get out of your contract early without paying a substantial fee. If you paid full price for the iPhone, then your chances of being able to switch carriers will be better. Only your carrier can tell you.
The other issue will be whether your iPhone is locked to your current carrier. If you purchased the iPhone from Apple unlocked, which I believe you can in the UK though I'm not certain, then no worries. But if the iPhone is locked to your current carrier, you'll have to get the carrier to unlock it, and will need to ask them for their terms.