So far so good for me with beta 4.
I believe people were experiencing some non-delivered call issues even before beta 3, and even before iOS 7 in general, but it was generally a fairly isolated issue not affecting too many people and usually much more rarely than it was in comparison to iOS 7 beta 3--if we are more or less back to those levels, then it sounds like at least the new issue that was introduced in beta 3 has been more or less resolved. Still, got to keep an eye on it and hopefully get more feedback from more people over more time.