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I'll go first~

It's the final hand of the tonpuusen, and you have a large score advantage. The dealer is making aggressive discards and is almost 100% aiming for tanyao. Can't gauge the value or other yaku of his hand this early, but you have a great hand shape that can still be opened up pretty quickly. There are two strategies for this hand, either getting rid of the to progress towards 7-8-9 sanshoku or getting rid of and moving for kuitan. The problem with the second one is that you'll probably end up in a wait where you could easily wind up furiten for the turn (6-9 of numbers or of pins leaves you furiten the turn someone discards the 9 if your hand is opened), so I'd choose for the first discard. The upper sanshoku is a better yaku to go for since the dealer is much more likely to deal terminal tiles while going for their hand, which you are free to chi away! Speed is critical since it's only the 4th turn and you could quickly end the session with a cheap win; hesitant playing could lead to trouble if second place forms a strong hand first.

A tanyao or pinfu would be more flexible than a sanshoku here, especially with that 369 wait. With the 5-sou, 346sou improves the hand toward that shape. Getting rid of the 89 before that isn't good because it would mean dropping down to 3-shanten and killing the sanshoku possibility. A penchan still gives 4 tiles for a perfect pinfu iishanten as well. Discarding a tile that adds 2 ukeire on top of the three-way is the right move here. And 7-pin is the better choice, because if you draw 3pin next, you get a 234pin mentsu and still have the 45678 wait usable.

Isolated.

Plus, you're bring room for a 7-8-9 Sanshoku, but don't count on it. Pinfu at the very least.

Kyuu wrote:

Isolated.

One of the first things I learned in mahjong was that isolated tiles are useful.