The information on the website doesn't tell me what's new, but here are a few things I've just discovered:
(1) You can now set custom date ranges. Great! I'm genuinely delighted by this, as it allows me to look at 28 days of data at a time. You know the whole "units of 7 days" rule right?
(2) More structured menus, and better use of shortcut keys. Very good.
(3) The ability to only publish a single campaign. Not sure why that would be useful, but it could be.
I'm sure there's more, but here in the UK it's 17:50 on a Friday.
Oh and don't worry if you can't yet upgrade. It appears they may be staggering the rollout.
Still can't set it to remember passwords though, which is mildly annoying.
And the custom dates are great, but I'd be happier if they just matched the Console (Month to Date is one I use frequently)
One problem - the new version appears to have wiped out all my comments on the Campaign and AdGroup level. That's kind of a pain. Does that normally happen between updates? Bleah.
By the way, release notes are here: [google.com...] (If you went to that page earlier today, and didn't see notes for version 4.0, you may need to refresh the page to see them now.)
More info on Inside AdWords, including the info quoted below, which I wish netmeg had seen earlier (sorry about that netmeg):
...To avoid losing unposted changes or comments, we suggest exporting an archive of your account before upgrading. After you've completed the upgrade, you just need to download your account and import the archive file to get your account up and running.
AWA
After some trial and error I found that if I selected "Everyone" during the install everything worked fine.
I have a hunch that if you don't choose the same preference you had on your last install that you'll have a problem.
In any event, posting this so if anyone else has the problem it might help them fix it.