Q: My iPhone 4s

iOS version?  Jailbroken?  What troubleshooting steps have you already tried?

Restore the phone in iTunes, first using a backup, and if needed as a new phone, without using a backup.  If restoring as new doesn't help, there is a hardware problem that needs evaluation by Apple or whoever provides iPhone warranty service in your county.

iTunes: Backing up, updating, and restoring iOS software

FYI, very few apps actually run in the background - the vast majority are frozen and don't consume resources.

iOS: Understanding multitasking

Restore it WITHOUT using the backup.  There may be corrupted data/settings in the backup causing the problems.  As I said in my last post, if restoring as new doesn't help, there is a hardware problem that you can't fix.