I am also having this problem only in the Mac App store... my iPhone updates just fine. But I've had 2 apps on my mac that I've been trying to update for well over a week, and keep getting the "unknown error (4)" I have already recently reset my Apple password... Apple actually made me change it very recently... and everything works fine on my iPhone, and I can sign into my Apple account just fine on my mac... it's only when trying to update in the Mac App store.
I have closed the app store program completely and re-started, that does nothing. I hated already resetting my password once because I had one I preferred to use that's easy to do quickly for me on my iphone, and they made me change it to something more complicated... I don't want to have to change it again just for this, that would be ridiculous anyway and should have nothing to do with this error, especially since they JUST made me change it like a month ago and since that new password works fine everywhere else within Apple! Closing/resetting the app store does nothing, my recent password does nothing... nothing makes sense with this error! What the heck is going on?? It's bad enough here on my mac, I can only pray it doesn't start acting up on my iphone also, that would really tick me off!
I just wish they would get this figured out and fixed ASAP!!!