This is a video on how to replace your LCD Digitizer Assembly on your iPhone 4, This is for Sprint and Verision phones, The At&t model is slightly different but you can use this video as a guideline just as well.
Step 1: Take out the Micro Star screwdriver and remove the Two screws at the bottom of the phone and slide the back off.
Step 2: Remove the battery by removing the Two screws holding the Panel on and unclipping the batter, Pry it out with the Guitar pic like pry tool. You need the Micro Phillips head for the screws.
Step 3: Remove the screws on the other two panels, one screw on the bottom holding the Speaker and Mic in place and then 2 more on the panel for the ribbon cable for the speaker and mic, then finally the top 5 for the board cover, 2 of these screws are of different size and need to go into the proper place when putting it back together.
Step 4: After removing the panels, carefully pry the ribbon connectors out with the pry tool, then remove the Speaker and Mic from the bottom, after that remove the 3 screws holding in the board with the Phillips head and lift the board up from the bottom and slide towards you gently.
Step 5: Remove the 4 corner screws on the inside of the frame and then the three screws on the sides of the frame so all together you're removing 10 screws, the corners are only for the corners and the sides are only for the sides. take a good look at how the assembly sits as well so you don't damage the new one when you put it in, also if there is a Bios CMOS battery (Sprint Usually) just pop it out it's only slightly glued. Now remove the Assembly.
Step 6: Place the new LCD Digitizer assembly into place and then reverse the Disassembly process.
Congratulations you succeeded in replacing your LCD Digitizer Assembly.