I'll jump right in:
1. You're right, those are the four forms of the imperative that exist in German.
2. All four forms are equally common - it just depends on who you are talking to. If you would normally use du, Sie or ihr with a particular person/group of people, then you simply stick with that form when using the imperative with them. If you're including yourself in the group (which you'll do essentially whenever you would say "let's" in English), then you use wir.
Note that you will include the pronoun when using the imperative with Sie or wir. For example: Bitte setzen Sie sich "Please sit down" or Gehen wir ins Kino "(Let's) go to the movies."
As you can see from these examples, the imperative isn't always direct or impolite - it can be used in polite situations as well.
3. When you're using the du form of the imperative, the -e ending is optional with most verbs - but not with all of them. With some, it has to be included. These are usually verbs whose stems end in -t or -d, or verbs whose stems end in most consonants followed by -m or -n. So for example, the -e must be included in Warte! "Wait!" and in Öffne das Fenster "Open the window."
For the imperative forms other than du, generally speaking, there aren't any new ending rules to worry about. There are of course always irregular verbs to watch out for, though.
Note that a full lesson on the imperative is currently planned for Level 2.
I hope that that is helpful!