gadgetfix, ebay seller.
As adviced, the front screen (lcd) is expensive, you can get the cheaper with only the glass, but it will require professional work. I would get the $90 piece.
For iPhone 4: Do not bother, it is not worse it at all.
The best shop for you - Apple Store - they will replace it in instant with new iPhone and you will get warranty as well.
Trust me: disassembling is difficult, but re-assembly even 2 times more complicated. You will be dealing with microscopic screws, spacers, tiny fragile connectors and etc.
At the end, I would be better off just giving it to Apple and be done with it.
Yes, you will need a full assembly: Glass + Digitizer + LCD. There is not way to reuse anything from old assembly - you will destroy LCD upon any attempt. Any video showing that LCD could be separated on iPhone 4 is fake, these folks just use separate components (not original) to make it look like they are geniuses.
I have iPhone 4 glass + digitizer - you can have it any time for $10 (paid $39 on ebay, so it will collect dust at your place).