I say, and with God's success: If one sits in the fourth rak'ah for the duration of the tashahhud and then stands up, the obligatory prayer is valid, and what follows is considered a voluntary prayer. However, if one does not sit in the fourth rak'ah, they must return to the sitting position; if they do not sit and instead prostrate for the fifth, the obligatory prayer is invalidated. Therefore, it is necessary for you to alert the imam when he stands for the fifth and has not sat in the fourth, so do not follow him. If he sits in the fourth, it is permissible for you to leave the prayer, as your prayer is complete, or you can continue to follow the imam in the fifth and sixth with the intention of a voluntary prayer. And God knows best.