...and mother is reduced to tears
You could see this as a big inevitable clash of power and personality, which is leading to a lot of struggles for the upper hand (which is pretty depressing as a weather forecast). He'll spend a hell of a lot of time on the naughty step until he is old enough to tell you where to put your naughty step and go to the pub with his mates.
Or you could break it down into individual problems to be solved as a team. Not "my 4 year old is always defying me and making me cry" but "my four year old wants to do X. Can I find a way to make it possible? Or can I find a way of offering something he'll like even more?"
Potential conflict can often be avoided by play - child says "get me that toy" you can say "no ,get it yourself" and watch the argument escalate, or you can say "oh, ok then" and get the toy, or you can fall down on the ground and say "I can't because I just turned into a JELLYFISH" and see if child wants to play jellyfish games. See what I mean?