I was handed an iPhone 4 with a busted screen
The repair took about 2 hours and required a magnifying lamp, tweezers, and slow, patient work, but everything worked out well.
I found that I had to remove the camera assembly to get the display to properly seat. Getting the retaining clip off of the camera was scary. Use fine-tip tweezers. It is latched on three sides, making removal difficult.
Also, it's REALLY important to get the display flex cables ALL THE WAY through the opening when attaching the display. They will flex and bend and fight you, but DO NOT PULL THE CABLES, or they will break.
As I removed each screw, I placed it on my work table in the exact same "location" as it goes back. That way, I knew where each screw went and didn't put the wrong screw in a hole.