Instead of four raka'ts, the Imaam performed five raka'ts, but made Sajdah Sahw.

Answer

If the Imaam did sit in Tashahhud in the fourth raka't, then stood up and completed the fifth raka't, the Fardh Salaat will be valid. He has to make Sajdah Sahw. However, if he had not sat in Tashahhud in the fourth raka't, and completed the fifth raka't, then the Fardh Salaat has to be repeated since it is not valid due to the omission of the Fardh sitting in the fourth raka't.