What should the worshipper (ma'mum) do if the Imam says the taslim (salutation of peace) by mistake in the second rak'ah of a four-rak'ah congregational prayer?

The worshipper (ma'mum) should alert the Imam by saying "Subhanallah" (Glory be to Allah) about what remains of the prayer. If the Imam does not understand, the worshipper may speak to him as needed. If the Imam does not comply, the worshipper should complete his prayer individually, or they may put forward one of them to complete the prayer with them in congregation, or each one of them may complete it individually. Their prayer is valid.