Support made a very thorough guide on how to activate 4.0:
so please check it out and make sure you followed the steps correctly.
Now, some users have encountered issues upgrading from the beta to the 4.0 release and there’s a solution for this issue:
1. Go to C:ProgramDataUnity and delete the .ulf file located there 2. Open Unity again and reactivate
1. Go to Library/Application Support/Unity and delete the .ulf file located there 2. Open Unity again and reactivate
If you purchased addons to your license and the editor hasn't been updated, please go to manage license -> check for updates.
If you followed the activation guide and tried the solution listed above and you still can’t activate - you might have encountered a bug! In this case you need to submit a bugreport.
Go to C:Program Files (x86)UnityEditor on windows or Applications/Unity on Mac and manually start the ‘Unity Bug Reporter’ located there from the installation folder. If you have more than one Unity installation, please make sure that it is the Unity 4.0 installation folder.
Please do it after a Unity activation fails as the editor.log is automatically attached and reveals a big deal about the issue. Additionally, if manual activation failed, please attach the .alf file you’ve downloaded to your computer during the manual process. As always, describe what steps you took when Unity activation failed.
Some users are running Unity 4.0 on Linux via Wine. Wine does support Unity 4.0 running yet. You can install Unity but not activate it.
Some users are using a non-pass through proxy (Unity should work with 'regular' proxies). In this case, try manual activation by disabling your internet connection before launching Unity. Follow the steps provided or check above link on how to activate manually.
Make sure your system clock is correct. The internet security certificate system relies on your time being in sync with global time. If your clock gets out of wack (say its 2013 and your clock says 2011) then unity's activation system will totally fail.
answered Jun 30 '13 at 06:40 PM