Myth - you'll be sent you jail for 5000 life sentences for hurting a criminal.
Truth - you'll go to court for some charge, and the jury/judge will dismiss you as being in self defence of others. 4v1 is no contest, really.
Okay say you have one guy in a headlock, so he isn't really "attacking you" while the others are. They obviously arent trying to kill you like in the OPs video. Are you allowed to to serious damage to the guy in the headlock on the assumption you fear for your life?
What about the other people in the bus who get sporadically punched. Whats proportional force for them? Any one of them could easily take them into a headlock from behind and do something.
Obviously this wouldnt be me doing it. I'm too weak.