well, youve got a few options...
1. You could throw a screen protector on and live with the cracks
2. bring it to apple and have them basically give you a new phone for 200 dollars
3. bring it to apple and make something up, hope the customer service buys it and swaps it out for you.
4. replace the screen yourself for 75-125 dollars (depending on where you purchase the parts from) and another thing to note is that since its an iphone 4 the glass is laminated to the LCD display so you have to replace both the display AND the glass, cant just replace the glass like you could on previous models.
hope i helped you out man