They have different functionality.

Autoplay just playing a song. It get COOL at all notes, press all HOLD buttons as they located.
Super Auto is trying score attack (get more points). The higher type of auto, the "smarter" it becomes.
Idk difference ot Type I and Type II, but they can switch HOLD button, is that means more points.

For example, line of this notes is ahead: ○(HOLD) ×(HOLD) □(HOLD) △(HOLD) ○○○○○○
So, autoplay will do this: ○(HOLD) ×(HOLD) □(HOLD) △(HOLD)=quadruple bonus => reset HOLD => press ○○○○○○
Super Auto Type I and II will do this: ○(HOLD) => reset HOLD =>×(HOLD) □(HOLD) △(HOLD)= triple bonus => press ○○○○○○
This way you'll get more points than before.

Super Auto Type III can do Fast and Late COOL and FINE is that mean more points. Example:
Line of those notes: ○(HOLD), pause, ○
Autoplay will press ○(HOLD) then pause and then ○ (when switch, hold bonus was +2930, 70 points left to get 1500 hold bonus)
Super Auto Type III will press Fast(earlier) COOL ○(HOLD) then pause and then Late COOL ○. This time HOLD was longer and it was enough time to get +1500 points hold bonus.

Super Auto Type IV can ignore notes is that mean more poits. Let's get back to first example. You know what autoplay, Super Auto Type I and II will do here.
Super Auto Type IV will do this: ○(HOLD) ×(HOLD) □(HOLD) △(HOLD)=quadruple bonus => press ○○○○○○ without release HOLD (use other buttons, arrows for exapmle => WRONG COOL=250 points for each button) or ignore them, if song have button restriction.

So you can use Super auto to see how to get more points fot song, however some songs can contain scripts to fail score attack.
(I hope you understand all that gibberish i typed)

Oh thankyou i got it, it helped me alot.
Hope this will answer the other to know these differences too.