I want to make it clear that this is not a theory created by me, but what I saw in the forum of the 4 horsemen of the apocalypse, but there are few people there, I believe that if posted here it would have more visibility and discussions, I do not fully agree with what it was said, but I think it's the most complete theory so far, official post link: https://www.reddit.com/r/Mokushirokunoyonkishi/comments/ofyzl0/about_arthurs_weaknesses_and_the_4_knights/

Okay so apparently, Arthur is expanding an empire that will hold humans only and I suspect that this is partly because his chaos magic which allows him to warp reality to a usually unlimited degree can only be defied by people who wield hybrid magics from multiple other races which are too chaotic for him to control. This makes sense as Chaos couldn't be released while both the Demon king and Supreme Deity, the two most powerful sources of non chaotic magic in the world existed.

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For thisvery reason, for the moment Arthur's sphere of influence is for the most part in the country where Camelot's main castle once was, as this area is where his power is at its peak due to the higher concentration of humans and lower concentration of other races, not necessarily because his powers would weaken if he went out of its borders but because he'd lose control over his own power if it were to be used in outside area, which could in turn damage reality as happened when he first took in the Chaos and even more when it became polluted by Cath's influence. Because of this he is maintaing a state of cold war with other kingdoms such as Lioness (ruled by Meliodas and Elizabeth), the Fairy Forest (ruled by Ban and Elaine), Megadozer (ruled by Diane and King) and the Demon realm (ruled by Zeldris and Gelda) all of which allow other species in and have non human rulers, while having human only kingdoms such as Edinsburgh as allies.

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Out of his past respect fot the Seven Deadly Sins, he has allowed their respective kingdoms to maintain their mixed population but he is slowly but surely expanding his sphere of influence by having his chaos knights (all of whom wield weaker chaos magics) drive out other races, while having the other kingdoms know that to oppose them is to oppose him, so he is practically goading them into an open defiance which would allow him to declare war on them while maintain the moral high ground, which could make more humans join his cause. The five remainig Deadly sins are aware of this situation, so they won't defy him openly until they figure a way to win, which they can only possibly do with the Four Knights, explaining why Sin is taking them Lioness.

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Considering Sin was able to identify Percival I'm guessing that he knows their traits and silhouettes, implying that he knows Tristan is one of the knights which makes me think that he is the one who sent him to go after the others. Considering the Conquest horseman (what the Pestilence one is actually called) is usually portrayed by the bible as the leader of the four and given his leading type personality I suspect that once he gathers the Four, he plans to declare open rebellion against Arthur while having his actions being condemned by parents, perhaps taking a retinue of his own hand picked holy knights serve as his lieutenants. In fact, considering Percival has assembled a group of friends can only be described as "protagonistic looking", I'm thinking each one of the knights will have a crew of his own which he'll lead, with their forces as whole forming up a rebel army that will oppose Arthur while informing the kingdoms of the situation so that they can eventually support them openly once their chanced of victory become something more than null.

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With this in mind the abilities of the 4 knights and their teammates will be something like this

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Tristan:

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1/4 human, 1/4 goddess, 1/2 demon

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His goddess purification (which we know was Elizabeth's specialty being capable of using it to heal others) and demon mutation in unison could make his body too chaotic for Arthur's chaos magic to warp away

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It could also give him a measure of biological control, making him an ideal Pestilence as he can heal and cause disease

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He also ought to have abilities simmilar to light and darkness wielded Mael's fallen angel

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I'm guessing he'll have a team consisting of the children of some of the old series characters now grown as holy knights

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I'm thinking that since he grew up looking up to the Seven deadly sins and with friends from multiple races he has taken Arthur's actions personally

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Lancelot:

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1/2 human, 1/2 fairy

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Being born from two beings who have consumed the fountain of inmortality could mean that there is no established limit for his potential durability and physical strenght, making him the perfect warrior

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As children he was physically weaker than Tristan, who had natural superhuman strenght but I'm thinking he may have surpassed him by having trained harder

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With a fairy's disaster magic enhancing the remnants of the fountain of youth in his body could allow him to control his cell's multiplication at will (explaining his not having an established appereance) which is why Arthur can't warp him as he can alter himself back to normal

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This would fit with the changeling motif

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His crew will consist on a brigade (cougharemcough) of ladies of the lake which will include no men because Lancelot himself was implied in the prequel to dislike other males (though ironically he dislikes being mistaken for a girl) and which will only worsen by his being raised mostly by women (Jericho and the Lady of the lake).

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I'm thinking he'll use his father's power to give out his powerful life force to others

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Percival:

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If my theory of his being Helbram's grandson (or son if the fairy biology is weird enough) is true then he is 3/4 human and 1/4 undead fairy

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Furthermore, from his white hair and ressemblance to Gil, Zaratras and Hendrickson, its possible Ironside has some druid descent; it would be interesting to explore his hipocrissy considering his more traditional believes in regards to race and gender norms

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The druid magic that kept Helbram undead (now being casted constantly by Percival's mostly human body) combined with his fairy magic (which enhances natural phenomenon) has given him the ability to enhance his own life force, with the fact that he is technically not properly alive but not rotting either meaning that he doesn't have the limitation of living body.

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Nobody can die in his presence unless he allows it so he can theoretically repel chaos magic

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His team will consist on the friends he made alongside his travels being formed by Donny, Nasiens and Anne

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It might also include the child of Gelda and Zeldris (Zelda?), whose existance I suspect was hinted by the fauxshadowing of Nasiens being a vampire, and being the Demon King's gradchild could be a stand in for Hades, whom the bible says is guided by the Death Horseman

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If so, maybe Zelda could fulfill a role simmilar to Elizabeth as an unoficial extra member of the team

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Gawain (if he is the starvation knight)

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I'm thinking he'll be either fully human or a member of the Celestials which have goddess and possibly human descent.

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The big ass sword he (or she in case the author decides to make him a manish looking girl 🤔) carries has the Sunshine grace (much like Anne's rapier might carry Flash) inside of it, meaning he won't be able to access its power fully at first like Escanor did but will be able to do so without having any ill effect

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As long as he hold the sword, he'd be too powerful to be touched by chaos

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I'm guessing his crew will consist in other wielders of the graces inhabitted (Ocean, Tornado and Flash) in weapons

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If Anne's rapier really has the Flash then she could always do part part time between both of their teams

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With all of this abilities the four knights could in theory enhance entire armies to make them capable of withstanding chaos magic