In four rakaat salaah,If a person join the Imam in last rakaat sitting(before making salaam),then after Imam make salaam, we stand up and recite surat al fatiha & another sura,then rukoo,then sujood,after sajda,we have to sit for tashahud or after completing two rakaah we have to sit for tashahud. thanks

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Whoever joins the obligatory prayer with the Imaam in the final Tashahhud, he should stand after the Imaam concludes the prayer and complete his prayer and he should not sit for Tashahhud except after having prostrated twice in the second Rak'ahs, and should not sit for Tashahhud after having performed one Rak'ah.

In case the prayer is a prayer of four units [like Thuhr, 'Asr and 'Ishaa’], he should stand after the Imaam concludes to perform two Rak'ahs, but if it is Fajr prayer, he should conclude the prayer after saying the Tashahhud and exalting the mention of the Prophet seeking refuge from the punishment of the grave and the torment of Hellfire, and from the trials and tribulations of life and death and from the evil affliction of Al-Maseeh Ad-Dajjaal (anti Christ), and he may supplicate after this with whatever he wishes. In case it is the Maghrib prayer, he should pray two Rak'ahs, recite Tashahhud, and then stand after the Tashahhud in order to perform the third Rak'ah.  

Allaah Knows best.