Hello, Tony—"At" works well in all those sentences. Although "on" could be used as a substitute for "at" in (1) and (2), I prefer "at." I don't know what (3) is supposed to mean, so I can't comment on it. "On" does not work as a substitute for "at" in (4).
To generalize, "on the top" is generally used when something is (physically) on the top surface of something else: on (the) top of the hill, on (the) top of the box, etc. "At the top" is used when something is being located, generally in a fixed, stationary manner, as being at the top of something, physical or not.