The Imaam stood up for the fifth raka‟t without having sat in the fourth raka‟t.

Answer

If the Imaam stood up for the fifth raka't without having sat in the fourth raka't, but returned to the fourth raka't before making Sajdah of the fifth raka't, then he should make Sajdah Sahw. The Salaat will then be valid.

In this same scenario if he completed the fifth raka't with Sajdah, then the Fardh has to be repeated.