I say, and with God's help: If you stand for the fourth rak'ah without sitting at the end of the third, the obligatory prayer of Maghrib is invalidated and your prayer is considered voluntary. However, if you sit at the end of the third rak'ah and then perform the fourth, the obligatory prayer of Maghrib is valid; because you have performed the final sitting at the end of the third, which is a pillar, but it is an incomplete prayer due to not saying the salam at the end of the third, and God knows best.