Q: My iPhone 4 suddenly switched off.

Could be a hardware problem.

If the iPhone does not power on when connected to the power adaptor and/or doesn't power on when doing a reset, more than likely there is a hardware problem with the battery or something else.

Try a reset by pressing and holding the sleep/wake button and the home button for 20 seconds (maybe longer) until the Apple logo appears, ignore the red slider.  Then turn back on. You won't lose any data doing the reset.

Then it may well be a hardware issue as Allan Sampson suggested. You may need to take a trip to the Apple Genius Bar.

Could be and in addition, hacking the iPhone (jailbreak) removes the built-in security included with iOS.

You don't conduct any financial transactions with this iPhone do you such as accessing your bank, etc.?

Under the Terms of Use of this forum it is prohibited to discuss jail breaking. By jail breaking your iPhone you void any warranty you may have, forfeit any support from this forum, leave your iPhone vulnerable to malware and risk bricking your iPhone should you attempt future software updates or restoring your iPhone - http://support.apple.com/kb/HT3743.

I would stop doing so with a hacked iPhone since all built-in security included with iOS has been removed.

Thanks a lot THE HUNTRESS.. It helped me..  Saved my iPhone.

The HuntressNov 11, 2012 7:06 AM Re: My iPhone 4 suddenly switched off.
Re: My iPhone 4 suddenly switched off.in response to Norway91

Try a reset by pressing and holding the sleep/wake button and the home button for 20 seconds (maybe longer) until the Apple logo appears, ignore the red slider.  Then turn back on. You won't lose any data doing the reset.