Whenever I start eclipse, I get this message that says I should buy Flashbuilder. There's a link that says Buy serial number for Adobe Flash Online but that goes to the Flex 3 page instead of the Flashbuilder 4 page...so it's not totally clear if it will work or not.
I decided to risk it and I bought the serial number for Flex 3. I entered it in and I got an error that says:
The serial number you entered is not valid. Click OK to retry entering the serial number.
So in other words, Adobe is telling me to buy a serial number, and giving me a link to buy it...but when I buy it, the serial number doesn't work!