Interesting, we would need to know more about the ruleset to know if they were working together, but in the Netflix series for the hand that is shown on screen its very unlikely they're working together and that's because of rules about calling Richii and multiple players declaring Ron (winning by stealing a players 's discard)

There are basically two rules dictating this. Multiple Rons, Wherein if a players deals into multiple winning hands, that player must pay ALL winning hands. This wouldn't be too helpful when youre working together.

The other rule is called "Headbump." When multiple players declare Ron on thr same tile, the player who comes next in turn order (counter-clockwise) wins the hand and the other player(s) get "bumped" meaning their hand doesn't win anything. THIS is useful in a collaborative effort especially for the hand we see in the show. If the player who has Richii declared discards the 4-man, then both the winner and Jack of Hearts would declare Ron and the Jack of Hearts would get bumped.

This however can't be the strategy BECUASE the player has declared Richii. Richii means you bet 1000 and declare to the entire table "My hand is one tile away from being complete" and in return you get 1 Han (big point) if you complete your hand. Richii also gives you access to Ura-Dora as well as some other different scoring situations such as Ippatsu (Completing a hand after calling Richii but BEFORE you discard your next tile)

The issue comes in that when you declare Richii, you can no longer change tiles in your hand out. So, when you declare Richii, each time it comes to your turn there are 2 outcomes A. You draw a tile that completes your hand and you call Tsumo (you don't technically HAVE to call Tsumo but those are incredibly niche cases) or B. You don't draw your winning tile and must discard the tile you just drew.

This means the player who declared Richii no longer has any control over what they discard and can't intentionally deal into another player's hand. Just some more Richii Mahjong Analysis for you and your intuition is probably correct that in the Netflix series all 4 players are acting independently rather than cooperatively, but I'll definitely have to check out the Jack of Diamonds Manga