Hi
This issue is usually caused by a damaged screen or loose screen connector
You will most will most likely need to replace the screen but I would open it up first and try reseating the display connector by following this guide
[guide|11755|new_window=true]
If reseating doesn’t work you will most likely need a display replacement which you can find liked below
[link|https://www.replacebase.co.uk/iphone-4-replacement-lcd-digitizer-assembly-oem-black|https://www.replacebase.co.uk/iphone-4-replacement-lcd-digitizer-assembly-oem-black|new_window=true]
Hopefully this helps
Any questions please ask
Thanks:-)