Q: iPhone 4 problem

restart phone

reset phone

reset network setting

restore phone as new

Operate iOS devices where the temperature is between 0º and 35º C (32º to 95º F). Low- or high-temperature conditions might temporarily shorten battery life or cause the device to alter its behavior to regulate its temperature.

Read here > http://support.apple.com/kb/HT2101

Hope that helps

The Basic Troubleshooting Steps are:

Restart... Reset... Restore from Backup...  Restore as New...

Restart / Reset

http://support.apple.com/kb/ht1430

Backing up, Updating and Restoring

http://support.apple.com/kb/HT1414

If you try all these Steps and you still have issues... Then a Visit to an Apple Store or AASP (Authorized Apple Service Provider) is the Next Step...

Be sure to make an appointment first...