The fourth chapter is too late. You need to do this within the first 10% of the story (by page count), anything more is too late.
If your MC does violent things for good reason, you need to give the reader the idea they are capable of this early on. At the beginning of a story, the reader is ready for anything -- magic, superhuman powers, aliens, gods, immortals, interstellar and intergalactic travel, shape-shifters, fantastic creatures and people like Wolverine. Anything! But their patience and acceptance fades by the 20% mark or so, if you haven't introduced something by then, it starts to look like Deus Ex Machina territory, like you are just making new stuff up out of the blue to move your plot along. You don't have to intro every fantastical element then, if you show magic, all kinds of future magic become a possibility for the reader. If you show one alien (or even mention them seriously), other aliens are fine, a whole culture of interacting aliens out there would be fine. One weak telepath allows far stronger telepaths, etc.
For violent things, you can be direct: Perhaps he kills a walk-on criminal, and has no regrets or qualms doing so.
Or you can be less direct, a man is harassing a woman, he steps in and pushes him off. The man pulls a knife, our MC doesn't flinch, they grapple and break the man's arm, then his nose with an elbow, the MC takes the knife and is about to kill the man before the woman cries "No don't!"
If you want to be completely indirect, you can at least show your MC in high-level martial arts training against rubber knife and paint gun-wielding opponents, actually delivering killing blows on dummies, as a kind of foreshadowing about his capabilities. As Galastel noted before me, that is a Chekov's Gun, you don't devote pages to this kind of training unless it is actually going to be used.
Likewise you need to demo the MC's commitment to good, that the government they want to overthrow is just as brutal. You need to show that the MC is on the side of good, meaning the MC engages in altruistic acts, the MC doesn't harm innocents for selfish gain (or at least doesn't engage in irreparable or irrecoverable harm; i.e. the MC is redeemable).
Likewise you need to show magic exists. There is no reason all these things could not be shown simultaneously in a single scene: A criminal uses magic to assault somebody and the MC altruistically intervenes, violently.
I am not saying all these rules cannot be broken, or have not been broken by best-selling authors. But breaking them risks getting rejected, and beginners should stick with the standard formulary: Introduce your fantastical elements early, and if your MC has any special powers or skills, characterize them early. While the reader is still open to anything.
The same goes for any dark side, indicate it early, but I would not show their dark side before their good side; first impressions are massively important here: Good first, and in your case, the dark side in response to darkness: The MC fights fire with fire.
You need to invent a new scene; perhaps on the boundary of Chapter 1 & 2, a way to end Chapter 1, start Chapter 2, or be Chapter 2. It can involve throw-away criminals that are gone forever after this scene, but you need to show it. Chapter 2 or 3 would likely work, but by Chapter 4, we are supposed to already have a good handle on the MC.