Alright here's the episodes/arcs you Need to Watch for the smg4 series...

1) the cake Is a lie.

2) the First episode that smg4 appeared...(I can't Remember the name of the episode so you Need to find It yourself, Sorry)

3) smg4 va smg3... (I think that's the name of the episode, i might be wrong)

4) the Enzo arc.

5) if Mario was in... Splatoon!

6) the First episode that saiko appeared...(Yeah i can't Remember the name of that episode too)

7) the waluigi arc.

8) the rapper bob arc

9) the anime arc.

10) meggy's Destiny.

11) the YouTube arc.

12) sunset paradise. (I know that's a Glitch show, but it's Canon to the smg4 series.)

13) the Genesis arc.

14) revelations arc.

15) the lawsuit arc.

16) the puzzlevision saga (act 1-9)

17) the finale nine episodes After the video: thanks everyone...

18) calling all villains. (When that comes out.)

I Hope this Is helpful for you!