The discarded 4 of Characters can be melded by Player as a Pong. This could also complete mahjong, but Player might be trying to build a higher scoring hand.

The left green button will accept the tile for Pong and play will continue with Player discarding.

The right green button will accept the tile for mahjong. You will see Win by Discard next.

The red button will reject the tile and play will continue immediately.