Wow, it is nice idea. This may be nice present for him. But I want to tell you that you should buy him another gift at the same time, because 4-years old boy, probably, won't understand your piece of paper

P.S. To avoid losing this btc, you should create kinda back-up procedure.
Of course man, maybe the paper wallet isn't enough for him at the moment ahah need to find also another little thing, thanks guys you have encouraged me a lot