Can't access Mail or Mail setttings on iphone 4 + iOS 6.1.2
First, force-quit Mail.app. To do this: Go to your home screen. Double-click the home screen. Locate Mail.app in the bottom bar that appears. Press and hold Mail.app until the icons wiggle. Delete Mail.app from the bottom bar by clicking red X.

Try again to launch Mail. If this doesn't work, reset the iPhone by pressing and holding power and home buttons for 10 seconds. Restart device once it shuts off.