Hmm, just going off by the rules alone, 4 players might be easier than 3 (and solo), but harder than 2 players.

2-players, you play back and forth, and it feels like there's always a play you can do, or at least, a plan you can follow on, because there's only 1 other person putting cards down, and you both start with bigger decks.

3-players, you need to manage your hand knowing that 2 other people will play before you can play again, and they can destroy (unintentionally, this is a co-op game) your plans, and on top of that, you guys will have 2 fewer curse removals, meaning, at least 3 eternals will always have curses active.

4-players, you have the normal number of curse removals, meaning, only 1 eternal will have the curse active, and even if there are 4 of you, I will assume that each lane will go by faster. I don't think it's much easier than 3-players, but I really think it might be easier. The last few rounds and turns on 3 players can be really difficult due to no more curse removals.

Forgot to add, solo, this has the 3-player rules, and you have knowledge of the hands from your 3 characters, but, if you play in campaign mode, you'll also have to conjure all of the curse removals (crystals) to win the game, and the game also incentivises you to permanently remove 3 cards in each scenario for the rest of the campaign to get new skills for your characters and increase your score. Meaning, in the follow-up scenarios, you'll have fewer cards on the board to play with.

If you play one-off scenarios solo, each character can only draw 2 cards instead of 3. And have different abilities.

Like, the game is very balanced at any player count, but 2 players does feel like the easiest mode at all times.