The iphone 4 and 4s is a bit tricky, lots of screws etc, plus the digitizer is glued then bonded and stuck to the LCD, so dont just buy the digitizer, there are some very good guides on the web.
if you want to do it yourself PM me for some life saving tips or I can do it all for you.."if I have time"
I can also do the ipad screens for the 1 and 2