The digitizer is not the glass, it's the hardware that's attached to the glass that allows your phone to read touch input.
You should research and find out if it's possible to replace only the glass. Sometimes the digitizer is glued to the glass and they must be installed as one piece. Don't use the fact that they are sold separately as an indactor that they can be installed separately. The iPhone 4 glass is sold all day long online (scams), despite it requiring the glass-digitizer assembly to be installed as a single unit.
Sent from my Nexus S