I have iphone4 but it shows 3G at top of screen is there a way to change to 4
Yes, you could try the sprint 4G product. It guarantees to transform your 3G iPhone into a 4G in no time. For more information on this application, visit sprint.com.
3G relates to the network it's using, not to the iPhone model. iPhone 4 isn't made for 4G, which AT&T isn't offering yet.