Few things you may want to try.
Hard reset. By holding the power button and the sleep/wake button for 10-15 seconds until apple logo appears (ignoring the red slider).
If that did not work, try restoring. You have 2 options from restoring.
Restore from New. This erase everything on your phone and is basically like the day you got it.
Restore from Back up. Sometimes not recommended, as the issue can carry over (not all the time but it is a possibility).
If none of the above fixes the issue, then you may need to set up an appointment with the genius and have them take a look at it. If they find it faulty, they will likely to replace your phone.
HP Pavilion DV2812tx, HP DV4-1504tx Windows 7 iPod Hi-Fi,
iPhone 4 32 GB Black