The glass on top is attached to the digitizer.. there is an lcd below. Does the lCD look broken also? Are the any dead/bad pixels?
My daughter just broke the glass/digitizer on her iTouch. I have received the replacement part and am awaiting the little tools to remove/replace.
If you go this route.. be aware that many posters suggest getting the glass and bezel and home button pre assembled since many people end up breaking the bezel since it's glued to the glass and the home button is notorious for being a PITA to get jut right.
That doesn't look like an iP4.. but if it is.. that's a whole nuther animal.. you need to go thru the back to repair screen and the LCD and glass are ONE part.http://www.youtube.com/watch?v=4mboB8p-sdw
I bought this for the iTouch...http://www.amazon.com/gp/product/B0041HPH88
Read the reviews.. some glass is nicer than others.. don't just buy the cheapest.
Or get Apple to fix it for ..I think it's $199.00
Local guys will do it cheaper.. and of course unless its done by Apple.. there goes any warranty you may have left.