in any case, any rating system has its drawbacks, it is better to confine ourselves to considering the systems that we know. this topic is about the Solo and FFA systems, the existence of which separately does not make any sense, since none of these systems does not solve the problem of cooperation at the stage of 4 players, so need to look for something more radical. but before moving on, need to optimize what is already there
The point, as made by MagicSteph above, is "why merge them?" People have the choice at the moment, if you enjoy Solo, play solo, if you enjoy FFA then play FFA.
I love FFA and hate solo. It's a completely different game. You need to gang up in solo, so it feels like getting a massive advantage at the start is discouraged, whereas in FFA you can always deliberately play for second.
In FFA, if you're sitting in a contest between 2nd and 3rd, with no chance of getting first unless you gang up, it's often a good strategy to attack the other lower points player to secure 2nd rather than try to gang up, whereas in solo that strategy doesn't work.
Full respect to people who like solo, but I don't, I hate it, I don't play it and it will probably see me moving away from FFA on chess.com for a while once I start getting up to that level, possibly for good (and it's the only reason I'm happy to pay for membership).
I agree with FlyingPlane that 2nd losing the same points as 4th discourages aggression in the early game. It seems wrong to me that someone who really dukes it out for first, plays the whole game through and only misses out by a narrow margin, gets the same as someone who hasn't had their morning coffee, hangs a bunch of pieces, gets destroyed early on and resigns at move 10.
Otherwise, robertcraigen has given the best answer on here. I haven't quoted it because of the length, but it expresses my view exactly.
If you want to play solo, go for it, just don't force me to.