oh tough tough one!!
I would flick the switch. Four lives could be saved by doing so. I would probably kill myself or drive myself crazy with the guilt of the other man though.
I wouldnt be able to push the man. If the weight of one person could stop the train, then wouldn't my weight suffice? And if I couldn't give my life to stop it (which I probably wouldn't be able to do) then how could I force someone else. I think this one I would literally be frozen with indecision.
I've often wondered about the person whose job it is to flick the switch in countries where there is capital punishment. How do you remove yourself from that action that kills someone? Could you rig up a machine that means the next person who inadvertently treads in a certain spot will release the injection and since they didn't know they trod in that spot they are not morally culpable for the death (do you follow me?) but then, isn't the person who rigged up the machine that made the next tread on that spot then the one with the moral culpability?