The iphone4 is a amazing device yet still damaged by water like every other electronics,you could maybe go back into the phone and make sure the connectors are in place all the way there is 2,I'm sure you already no that,I would pop them off and blow out the connecters then plug them back in and make sure you here a click that will insure that they are all the way in place.Also you might even try replacing the battery the screen takes allot of power to use so maybe your battery is damaged.
Try this and let me no how it goes.GOOD LUCK!!
P.S. you can also buy 100% water damage repair kits.