This is very original, in terms of stick series. I like how you incorporated comedy with action into this series and took a very unique turn that very few others have done on newgrounds. That being said, I can't help but wonder if Kung Fu Panda or Karate Kid gave you any inspiration. Oh well. Most newgrounds movies are created upon inspiration of another work, and those works by previous works and so forth. I'd guess Kung Fu Panda as Po's so-called "fat" and "soft" style corresponds to this stick's "fart" style. But again as it's pure speculation, I can't be sure, so that's why I'm asking.
So we all know that his finishing move is the so-called "fart". Element-wise it is technically air. It can also be applied as biochemical weapon, which fits the "weapons" category. And Being able to emit a large amount of toxins and store a large amount of metabolized gas as well as release is in a deadly form is an ability of sorts. Which is fun in terms of comedy. Definitely original.
What I think you could improve on:
Well, for the first thing, I would hire voice actors. Voices can definitely add a lot to the dramatic effects and could jerk us awake when we're bored. And there are times in the videos when we do get bored.
For example, although your series did take a unique turn, it does what every cartoon on television, parody: make a dramatic badass entrance for the protagonist and then muck it up by making him fail at every trial. It's gimmicky. It's corny. It's overly played. It's been done over and over and over that when I saw the trials, I KNEW it was going to be him failing at every single one and yes, I knew it was going to be a fart as I can related it to a previous episode. If you can predict it, it's BORING and you should take some twists.
For example, the "Larry" series could be looked at if you haven't already. Although there are a lot of gimmicky references and recurrences that do make me sick, there are parts that are hilarious, like in episode 2 where Larry slaps his dog with a stick and accidentally chops his foot off and he runs to stimulation of alcohol. THAT is unique. Because I did NOT predict that and even if I did have a feeling, it still made me laugh so hard. Your series 4 did not, really. I predicted what was going to happen in every scene. So I suggest you can add some twists, something no one can predict. Even one per episode could be good.
Your silly facial expressions is a good twist. That's a start. Which is why I actually think voice acting could help to the comedic effects. You could make some more good work on that.
You can also add humor to the "epic/strong" characters like Red and Pink. Let them have silly habits. Let them have secrets or something silly that no one could predict. You could take a turn at poking fun at them. Let them mess up, too when you expect them to win.
So in essence, make the series more unpredictable. That's the good and bad of the series in a single sentence - it is original in theory and unique, but has too many overplayed elements. Add some more twists. Whenever you create a new scene, ask yourself, "Is the audience going to predict that?" And if you watch it and think you know what's going to happen even if you're the artist, throw it out and rewrite.
All epic series eventually fall because of they run of out ideas. Keep the series fresh with new twists and angles and throw things in our direction that completely blow our mind. If you think of immediately, it most likely has been done a million times. And when you think even more outside of the box, and give your next episode that element of twist beyond the normal twist, I can safely say then I'll give you the full 5 stars.
Best of luck and wishes! Looking forward to the next Stickman Can't Fight episode.