You may never know why, but the best solution is to just re-download minecraft itself.

You can delete minecraft's main folder and then run Minecraft and it will reinstall everything with B 1.4 in it, to find the folder in windows 7 use the windows start and search %appdata% and open roaming and delete the .minecraft folder on mac go to user/library/application support/ and delete the minecraft folder, and run minecraft and as said above will reinstall

If it stays at downloading packages, then just re-open Minecraft. If that still doesn't work, go to (in Windows Vista/7) start and search %appdata% Open Roaming. Click on .minecraft and just copy the saves folder to somewhere else on your computer. Afterwards, delete the .minecraft folder and go back to minecraft.net and install it again. It will install everything including the current update. Go to the new .minecraft folder and delete the saves folder. Afterwards, copy the saves folder from before you deleted the old .minecraft into it and you are done.

If you purchased minecraft AFTER the release of the beta version, you may not have the future updated versions.

do what RDRplayer says

Run %Appdata% then you will see a folder called 'Minecraft.' open that and look for 'Bin' delete that and restart your pc. Once it is started up load minecraft and it will update!