6.7 Furiten

Having already discarded one of your own waits is called furiten.

In furiten, you cannot ron. You can reach agari only by tsumo.

Toimen discarded 1-sou before riichi.

1-sou is one of toimen’s waits. Once you discard even one of your waits, you cannot ron on your other waits either. So toimen cannot ron even on 4-sou.

Discard 4-sou.