how to get fixed the back broken glass of iphone 4 and how much the cost

Take the iPhone to an Apple Store, if there is one on your area. Many Apple Stores can replace the back glass; they'll have to tell you the cost, though I believe I've seen it reported as costing $59 in the US. If you don't have Apple Stores where you are, you can find out how to get service through this page:

http://www.apple.com/support/iphone/service/faq/

You can then ask whether the service provider can just replace the glass or if they'll have to replace the entire iPhone.

Regards.