I'm glad that it turned out fun for you and the kid! When I learned (like 3 weeks ago) I was 13 and I was still getting really frusterated so I can't imagine how frusterating it would be for a 3 to 4 year old!
I'm going up soon with my best friend/neighbor and his little brother, who is 7, and I tought his little brother Bmx and Skating (I can actually bmx unlike snowboarding) and he progressed so fast! He can already bunnyhop 180 and can ollie, nollie, manual for a good distance, and can almost kickflip! His older brother, my friend, wants to ski, and the little brother wants to snowboard. He's getting lessons and I'm sure he'll progress faster then an average 7 year old would, being that he is completely fearless, crazy, and has major skill on a skateboard! After his lessons are over I know he'll be hungry for more and I'm not good enough to teach him but I'm good enough to play games and to tell him tips after his pro instructor leaves. He's super stoked and so am I!