My goodness, you have every right to be concerned. I am a grandmother of two girls, ages 3 and 6, and I would absolutely die at the thought of one of them opening a 2nd floor window, let alone taking or knocking out the screen. We also live in a 2-story house. But first, let me just say I cannot imagine a 4-y/o knowing he/she needs "fresh" air from the window. What I can imagine is a child this age doing what he/she is told or imitating what they have seen someone else do, perhaps on television, with no knowledge of the danger it poses. Since the windows are obviously well within his reach, my recommendation would be to childproof the windows. You can either secure them with nails or replace them with childproof windows which opens from the top only. I realize children today are more independent than they were years ago, but this should not be taken to mean their comprehension skills are so much more advanced. Also, I would let him know that in the future if he needs fresh air or wants a window open, he should ask for help from an adult, but he must never touch the windows. I am old school, and as such I fully believe there is such a thing as being too young to know. These days children die too often from freak accidents of this nature. I am glad you came forth before an accident occurred, and even more glad you discovered he's been playing at the upstairs window. Today, a lot of parents are too laxed to realize that although their children act grown-up, they are still only children...our babies. Honey, secure those windows immediately, and don't encourage that baby to seek fresh air on his own at the young tender age of 4.