iOS 7.1 beta 4 seems very stable - performed a full wipe and restore as new and loaded all apps from scratch and set them up as new (no iCloud data backup) on my iPhone 5S and iPad 2.
I've had zero app or iOS crashes thus far. My biggest gripe now is the occasional lag scrolling various menus (settings, settings-> general, safari-> bookmarks are laggy/jumpy occasionally) and the cosmetic issues that are still present or new such as the new + add contact icon in the keypad dialer isn't centered with the number delete icon on the right or the brightness slider in control center isn't the same length as the volume slider or the star icon in safari bookmarks needs to be moved to the left one pixel to center it vertically with the history icon below it - there are lots more polish issues like this.#
iOSBry.