- A step by step guide is on this link below: Installing iPhone 4 Display Assembly
- Check the battery out, try replacing it but I don't think it could help much... Most probably problems with the logic board. Usually it acts like that after water damage...
- Connect it to the charger, and try to start it up, it should show the apple logo if the logic board is ok... If it doesn't react, remove the battery and plug it in the charger see if it starts, if no apple logo most probably the logic board is dead. *Also try to connect to a PC with the usb cable and hold the home button, it shoud start in recovery mode, or put in in dfu mode and wait for the sound of connecting to the computer.