press power button and home button together for 8 sec

You can watch video on Youtube, there are plenty of videos showing how to replace the iPhone 4 LCD screen digitizer. It is better to replace the lcd with touch glass together. If you want to DIY, you purchase a new iPhone 4 LCD Screen Digitizer Assembly from ebay or online store, it will not cost much. Hope you can get your iPhone 4 replaced well. Good luck.

Go to your local repair shop for help, they will replace your screen

Hey, what is your iPhone 4 model? GSM or CDMA ? You should buy the LCD Screen + Glass Assembly, not a single glass. Because the glass and LCD Display are connected very firmly. It is very difficult to separate them.

Here is the info to show you what is the difference between CDMA Version and GMS AT&T iPhone 4 LCD Screen

Good luck !