If only 4 companies can afford to be in a market because they strongarm any competition out because they have money to burn due to collusion (which this can arguably be framed as)?
Yes. By practice, oligopolies (the practice of a market being dominated by 2+ companies) can be deemed a monopoly in practice. The courts would consider how the practice leads to a complete strong-arming of competition out of the field due to completely unreasonable feasible costs that other companies cannot afford (to quote: "Collusion might involve two rival competitors conspiring together to gain an unfair market advantage through coordinated price-fixing or increases.") It's something NO major company wants to have investigations about, because it's rather easy to come to a conclusion if say the president of Microsoft had any call with Tim Sweeney before the initial decision to support Epic.
If a group of super-rich companies decide to suddenly disrupt the market and drop the average stocking fee from 30% to 12%, suddenly it can and will have an outreach. Companies which require that 30% to remain profittable, some of which may be subsidiaries like... let's say GOG? Those companies will be forced into a situation where their profits are cut by over half.
Let that sink in. Profits would cut more than half in that method (60% actually). Unless that company's making a massive source of money from other sources (IE: Gamestop, who doesn't give companies the cut but pockets it due to attempting legal loopholes through resales), a LOT of companies in that end will SINK under the profit margin line considering all the money/deals they need to invest in order to even get the game offers. This is excluding website fees, maybe server fees, employee fees for CS or for ensuring the business runs smoothly, so on and so forth.
This especially affects the online markets, again, because odds are their main product is going to be those games. Again, right now Epic Lame Store is
losing money and will be expected to lose money
for at least a decade after the store launched, and that's in epic's own estimates.
Now let's say they aren't Epic Games, who have a nearly bottomless well of money stored from video games over the last 2 decades. Let's say they want to start up a business in the game distribution market. This market would be feasibly unaccessable, because a small group of companies are all offering publishers 12% cuts. But in order for your company to profit, you need let's say even a 25% cut instead of Apple's 30% (Apple and Steam of which includes other functions could argue that 5%).
No publisher would want to do business with you if you're asking for a 25% cut instead of the 12% cut. You can try to garner a fanbase, attempt to overcome the deficit you'd be taking each year, but you'd probably collapse underneath the lack of a userbase sooner or later. This would fall under a
monopoly in that these companies are preventing other companies from even being able to get off the ground because of the fact they could afford to throw way too much investment into a cash guzzling storefront until they stabilize nearly a decade later.
Other companies have shut down functions for less loss than this per year. And I'm not limiting that to just gaming distribution websites.
The only sort of legalised monopoly is called a 'natural' monopoly. It's usually done because of a superior product wherein people don't care about higher prices because of what's on offer is often done so with higher quality. Also it can happen simply because a company has access to an extremely unique or limited resource (including geography - see Phone/Power Lines). In short, this sort of monopoly is supposed to occur rather naturally.
However, it's not like this falls under a 'high-investment' Start-Up cost, has limited resources, etc. A high startup cost caused by competitors is not 'natural', it's artificial, and rather easy to argue. And digital games are not limited. This whole definition would be REALLY easy to argue in court. They're arguing 30% is attempting to keep too much money, but 12% could be argued in the opposite way.
Edit:
Let's say that you're trying to sell to me a website for $500. However then suddenly @Chary decides to offer me a website for $200. You need the $500 in order to remain viable in business, or else you'd begin to fall under a reasonable profit margin. Chary can afford to sell websites at $200 because of GBATemp and writing articles makes them enough to recoup (example, not true). You wouldn't be able to compete because why would someone want to spend $500 on you (30%) when we can spent $200 on Chary (12%).
This is a good way of looking at this whole situation. (OFC I will yield, maybe you only need 20-25% to remain profitable for a lesser company. But that would only make a point that 12% is monopolistic competitive pricing attempting to strongarm out any competition).
Of course, I doubt you'll bother reading all this, and probably ignore bits of what I say and argue something I've already explained rather thoroughly.