The musaafir Imaam performed four raka'ts Zuhr instead of two.

Answer

The Salaat of the muqtadis is not valid. They have to repeat the Salaat. If the musaafir Imaam had sat in Qa'dah Ula (the first sitting after two raka'ts), then in error performed another two raka'ts, he was required to make Sajdah Sahw. His Fardh is valid, but discharged defectively if he did not make Sajdah Sahw. If he had not sat after two raka'ts, then his Salaat is also not valid. If he had intentionally performed four raka'ts, but sat in Qa'dah Ula, then although his Fardh will be valid, he has committed a major sin which the Fuqaha say is punishable with the Fire of Jahannum.