Hello!

When I've encountered this issue a replacement display assembly has always fixed the issue. How recent was the replacement installed?

If recently then get in contact with your provider and tell them of the issue. Before you do this it can't hurt to try and reinsert the Flex cable.

Please do this first to ensure this is not the issue.

Follow the iPhone 4 Display assembly replacement guide but not all the steps as not all are required.

Follow steps 1-7 and then ignore steps 8,9,10,11 and 12. Follow steps 13 and 14. Ignore step 15 and 16. And then finally complete step 17. Then go backwards and reassemble your iPhone.

Please get back to me with the results :) If unsuccessful then either get in contact with your provider if the assembly is within warranty or buy another replacement.