I believe that Flash Builder 4 beta 2 is supposed to come out Monday based on some recent docs, no clue on anything past that. My team has been working with nightly SDKs and prerelease Flash Builders for the last few months, and while there definitely have been issues with fluctuating interfaces, I feel that the quality has been consistently improving.
You can track current bugs at http://bugs.adobe.com/jira/secure/Dashboard.jspa and download nightly SDKs at http://opensource.adobe.com/wiki/display/flexsdk/Download+Flex+4
The june beta was the first public release, the second public release will coincide with the Adobe Max Conference (my guess is after the conference you will be able to download it).
I am not sure what you mean by "outside" developers, the beta1 was public and there will be a public beta2 and my guess a third. There are several resources available for developers through Adobe including information on beta and released products.
My concern is that the questions you ask seem rather strange, unless the concept of "Beta" is foreign to you.
How many bugs were found = http://bugs.adobe.com/flex/
How stable is it = beta1 was twice as stable as max preview, beta2 is twice as stable as beta1, but this is a subjective evaluation, that is why the product is in beta usability is regarded as acceptable but there could be issues yet to be found..
Provide a real time status update ? - this is a joke question i assume.
If you wish to evaluate Flashbuilder as a viable development platform then the beta2 is definitely stable enough for that.
Our experience has been very positive. We download a compile a nightly build every morning, do a build of our application and then quickly fix any issues that builder shouts about. This has been happening less and less. Since the big rename session a few months ago things seem to have become much more stable, and most of the changes seem to be additions to functionality rather than anything else.
It's true that some of our code isn't taking advantage of some recent api changes, mainly in the area of extra styling in the default skins, but thats fine as we don’t use them anyway.
I'd say the advantage in starting using 4 now is WELL worth the small effort in tracking changes to the SDK. I've found that having to be aware of this changes has also given us a better and deeper understand of the sdk in general as we've all had to keep a close eye on what's been happening with it on a daily basis. We're more than happy with how the process has gone.
The one thing that has been a huge help is the amount of dialog coming from the dev team. If changes are about to happen, just keep an eye of the wiki and the forum and you'll be warn well in advance in most cases.
Lol, having said all that we have just run into an issue with resource bundles that it's my job to look into today, our last compile of our application just couldn’t find any of the .rpc classes. (anyone?????)
My adive, go with the nightly build, keep a very close eye on the wiki, dont be afraid to contect the dev team and be prepaird for the occasional Sunday working to fix some chages.
All in all a very good experience here
tinylion development & design