I am kind of getting into the iPhone development thing so I joined the developer program a while back. I ran Beta 3 for a while and I am not running Beta 4 on my iPhone. I found Beta 3 to be quite stable, but Beta 4 I have had more issues with. Before starting out with Beta 4, you will need to download iTunes 8.2 Pre-Release. The install didn’t go as smooth as Beta 3 did and I seemed to have a minor issue restoring my phone but everything appears to be working ok. As with the other Betas, the phone is noticeably faster in places. However, about once a week, I will find that my phone is off. After attempting to turn it back on, it will give me the Connect to iTunes screen. This can be a big pain. Connecting it to iTunes will make it magically start working again. However, it happened to me today at work and iTunes was not an option. I tried multiple times shutting it off and turning it back on again and eventually it started working again. Other issues to note is that sometimes it doesn’t seem to pull down mail even though its scheduled to push on some and retrieve others every 30 minutes. It doesn’t do this always, but it definitely does happen from time to time. All in all, Beta 4 is usable (much better than 1 or 2), but it is not perfect. My recommendation is to stay on Beta 3 if you want a beta and wait and see what we get with Beta 5 (which I am sure will be pretty soon).