How deep do I cement a 21' tall, schedule 40 galvanized steel pole?
4 feet is probably an overkill. The building code in my area requires 30 inches for deck supports where the load is much greater.

You don't need rebar or a tube, just pour Quikrete mix it with water in the hole while holding (or somehow supporting) the steel poles. In 10 - 15 minutes the concrete will set enough so you can remove the supports.