First, make sure the phone is COMPLETELY dry. I would take off the rear panel, disconnect the battery and leave it alone for several days. If you want, you can put it in a bag of rice to help soak up any moisture.

Try a soft reset of the phone. On your iPhone 4, press and hold both the Sleep/Wake button and the Home button for about ten seconds. The phone should restart.

If that doesn't work, try a restore.