I tried to make a safe-upgrade on a friend's pc where i had intalled BT4 on hard disk. It dowloaded about 100M. First i had activated almost all repositories, but had not fixed the gpg issue at this moment.
Finally i ended up no more being able to open any new applications when finished, even konsole, but aireplay-ng was still running (i was using it at the same moment)
I rebooted and..had by default immediately the fmwm desktop! Nothing else left it seems.