If you want to know how to replace iphone 4 screen, there are a couple of things you need to know. The Apple iPhone 4 front glass, touch screen, and Retina LCD are fused together in the factory and assembled as one part.
To repair iphone glass, it is not possible to replace only the glass using OEM replacement parts. The proper repair for a cracked iphone screen, touch screen problems, or damaged LCD is replacing the iphone 4 screen assembly.
The cost to repair your iPhone 4 screen is listed below and includes the OEM replacement part, labor, and a 90 day warranty.
Make sure you iPhone shows up when connected to iTunes or you may have other issues.
Since your checking your charging port, Why don't you backup your iPhone in iTunes before bringing it in. Better safe than sorry.