When I was not yet 4, I spent about a week with my aunt and uncle who lived in a third-floor apartment in a small town. One day, I was with my gregarious uncle in his store and he got busy talking to people, which bored me silly. So I tromped out of the drugstore, walked a block or two down the street and into their building, and tried to get back to the apartment. Unfortunately, I couldn't reach the "3" button in the elevator. While I stood in the elevator pondering how to grow tall enough to go higher than the second floor, my frantic uncle came racing down the street, relieved that I hadn't walked into traffic.
Yep, you can't turn your back on a child, even for a second.