What is the ruling regarding a four-rak'ah prayer (like Asr) led by the Imam who performed only three rak'ahs by mistake, and no one alerted him until after the Taslim and Tasbih?

If the Imam performs the fourth unit of prayer after being reminded, and the interval is not long, then the prayer is valid. However, if he does not perform it, or if the interval after being reminded is long, then he and his followers must repeat the prayer.