If the traveler forgets to recite only four rak'ats without kosr, then it is necessary to see if he has sat after two rak'ats . In this case, the first two rak'ats will be obligatory and the last two rak'ats will be nafl. And because of that mistake he has to do Sahu Sijdah. Hasan Basri Rah. He said that if a traveler forgets to perform four rak'ats of Zuhr prayer, then Sahu will prostrate. (Musannaf Abdur Razzaq 2/541) So in this case he has to perform two rak'ats again. ( Maraqil Falah 231 ;