You either have to:
1. Pay a fee to get it fixed or purchase a new phone.
2. Attempt to dry out any water that is left in there that could be causing shorts.
Many forum users here would advice you choose the first choice, but I think you should leave it powered off, use a blow drier, maybe try leaving it in a bag of rice for a day or two. Possibly you could seal a few pouches of dessicant (that stuff that comes in a bag of beef jerky) in the bag with the rice and your phone.
Turn your phone off & try don't try switching it on...doing so "may" short things out. You can "try" putting the phone in a ziploc bag of dried rice...Silica Gel Pacs work best... for 3 ~ 5 days, then hope things work. Apple does have an out of warranty replacement program, & the cost for an iPhone 4 is US $149.
Use the rice now, then get the Silica Pacs.
The chances of the rice/Silica Gel pacs working is not great.