four rakaats

Q: Our imaam offered 3 rakahs instead of 4 rakahs in asr farz Salah and ended the salah. No one interrupted him for his mistake and all the people ended offering 3 rakahs, then people said it was only 3 rakahs completed so the imam repeated the 4 rakahs again and every body prayed 4 rakahs again.

My question is if a person is behind a imam in the above case and is aware that the imam has made a mistake can he individually continue the same Salah and complete it ?

Q: One common mistake I make in salaah is sitting in the 3rd rakaat when reading a 4 rakaats salaah, please guide me whether I need to make sajda e sahw?

Q: While performing four rakaats Sunnah I forgot and completed two rakaats with salam. My question is shall I repeat all four rakaats or just offer two rakaats.