Q: My iphone 4 has a black screen but is on.

Hello Makintime6,

Welcome to the Apple Support Communities! Oh no! I’m sorry to hear that you are experiencing issues with your iPhone 4. The following resource outlines the specific troubleshooting steps if you see a solid black screen on your iPhone:

Get help with the screen on your iPhone, iPad, or iPod touch - Apple Support

http://support.apple.com/en-us/HT203039

If you see a solid color screen (black, white or blue)

1. Plug in your device to a power source. If you see the low-battery screen when you plug in, let your device charge for at least 10 minutes (20 minutes for iPad) before using it.
2. Reset your device while it's plugged in.
3. If resetting doesn’t fix the issue, restore your device.
4. If you still see the issue after you restore, contact Apple Support.

Since it also sounds like you have a hardware issue with your device, you may want to skip the reset step and move on directly to a restore and contacting Apple Support, if needed.

Cheers,

Matt M.