If you pray behind someone who is completing, you must pray full even if you only joined him at the fourth Rakah.

I know that Qasr is shortening four to 2 rakaahs but if im praying isha eg behind a local imam who’s praying four rakahs but im on Qasr but i still have to follow the imam and pray 4 rakaah in full… Correct? But Shaykh What if i join the jamaah on the second rakaah in the first tashahud what will i do then? Would i complete four rakaah? Or say tasleem with the imam?