This isn't immediately actionable from the MobileFrontend side, but I'm filing it anyway as a reminder that this is happening for some users.
On iPhone 4/iOS 6.1.3, tap edit on an article section that's reasonably long (a few paragraphs). Tap in to make an edit and then tap done on the device keyboard. You won't see the bar with options to cancel, preview, or save at the bottom of the screen, as you do on earlier (6.1.2) and later (6.1.4) versions of the OS. You have to manually scroll all the way to the bottom of the section (which can take a long time on long sections) in order to see and use cancel/preview/save.
Unfortunately, 6.1.3 is currently the latest version of the OS available to iPhone 4 users, so until Apple gives them a software update, editing is going to be slightly broken for iPhone 4 users like me :(