I have an iPhone 4. When I save certain images or use certain presets the saved image is missing a rectangle on the lower right.

This seems to be a memory problem that occurs from time to time on an iPhone 4. The iPhone 4 has somewhat limited memory compared to more recent devices. Things you can do to try and remedy this (try one or both of them):

1. Restart your device. This will make sure no other apps are holding onto memory.
2. Try a lower (“Small”) output resolution: