How should one act who intended to pray Maghrib with a congregation, but it became apparent that they were praying Isha by way of الجمع التقديم (combining the prayers by advancing the later one) and had already completed four rak'ahs?

The most authoritative view is that a difference in intention between the Imam and the congregant (Ma'mum) does not affect the validity of the prayer. So, if you pray behind someone who is performing the 'Isha (night) prayer while you intend to pray Maghrib (sunset) prayer, you should sit for the Tashahhud (definition) in the third Rak'ah. You then have the option to either leave the Imam and say the Taslim (definition) (salutation of peace), or wait for him and say the Taslim with him. If the congregant finishes their prayer while the Imam is still praying, they have the choice between leaving him or waiting for him, and waiting is preferable. However, if one prays Maghrib behind someone performing Dhuhr (noon) prayer, and the Imam stands for a fourth Rak'ah, the congregant is not permitted to follow him; rather, they should leave him and perform the Tashahhud. It is permissible for them to lengthen their Tashahhud while waiting for the Imam.