Like most Apple updates, we suspect that the new beta 4 includes several stability and performance enhancements that might make the beta more pleasing to use, if you are testing it.
We've heard from some early beta testers complaining (see article comments from this April 1 post) about the number of issues they were experiencing with the previous releases.
It is too early to tell what kinds of changes have been made in beta 4, but we suspect that it will not take long for people to discover them.
Read CNET's additional coverage of iPhone OS 3.0:
A very early review of iPhone OS 3.0
iPhone OS 3.0 hands-on screenshots
iPhone OS 3.0 event video added to iTunes Store
iPhone OS 3.0: What you need to know
Developers and beta testers, let us know about your experiences with this latest iPhone OS 3.0 beta in the comments.