"But I am still young!... or
"But I am only a child!... or
"But I am only 4 y.o.!... or
... so I don't know how/that it's wrong/what to do/can't control myself!"
He must have learnt it from us. Sometimes we explain to him that Nicholas is still small, and he doesn't know that certain actions are wrong/not nice and we have to be patient to teach him.
Sometimes when E scolds him for his tantrums or bad behaviour, I may whisper to E that he is only small, so he can't control himself. When I lost my temper with him, sometimes E will also defend him in a whisper, more like a reminder to me that he is only 4. So he must have overheard us.
We have never spanked him or even hit his hand as a form of discipline when he does something wrong. Besides time-out, we do send him to his room or take away the objects that are the centre of conflict (usually toys which he fights over with Nicholas). And I believe it is because we have never spanked him, that he doesn't even need spanking to know that he is being disciplined.
Sometimes after he has done something naughty, like kicking Nicholas or snatching his toys, E or I just need to walk over and use a harsher tone to tell him that what he did is wrong and he would start crying. We do recognise that some episodes of crying were just drama he put up to gain some sympathy points but other times, they were genuine tears of remorse and even embarrassment of his own behaviour. We then react accordingly. But over time, I do think that he has learnt what is acceptable behaviour in many ways and what are strictly no-no, even though he will continue to challenge the boundaries. I see that as a positive development and it can only be wonderful as he learns more about himself, his feelings and how to deal with people around him.