Definitely check at your local resort for learning programs specifically targeted to his age. Our local hill has a really great half-day and full-day program. Below is a link to their site, it will give you an idea of what I'm talking about. The sessions are specifically geared to children, mix in lots of fun stuff to help with the attention span like games during their sessios, etc. It keeps it really fun while also teaching them to ride/ski. As you point out, a 4 year olds attention span isn't going to work with a standard 1-hour lesson on falling leaf, etc.
When my nephew was 6 or 7, he wanted to learn to snowboard. We put him in the all-day class and he loved it. The next year we bought him his own board, boots, and gloves and he started doing regular 1-hour lessons from there.
Kids Ski & Snowboard Progarms | Mad River Mountain