Must the Maghrib prayer be repeated if I prayed four rak'ahs behind an imam, thinking he was leading the congregational Maghrib prayer, then it became clear that he was praying another silent four-rak'ah prayer?

If the aforementioned person is performing a four-rak'ah obligatory prayer (like 'Isha'), then it is permissible to follow him according to some scholars. When the Imam stands for the fourth rak'ah, you have the choice between leaving him and saying the taslim or waiting for his taslim. If you follow him into the fourth rak'ah due to forgetfulness or ignorance, your prayer is valid. However, if you intentionally add that rak'ah, your prayer is invalid.

If it becomes clear that the aforementioned person is performing a supererogatory (nafl) prayer, it is also permissible to follow him. Your ruling is not to follow him in adding the fourth rak'ah. The prayer is valid if that addition was due to forgetfulness or ignorance, but it becomes invalid if done intentionally. There is no harm in you repeating the Maghrib prayer if you believe it was invalid, as this falls under precaution and avoiding scholarly disagreement.