ANSWER:

This makes no sense! 

Why did you stand when you knew he was standing for the 4th?

If you thought he was standing for the 4th yet still followed him in that, your prayer is invalid.

QUESTION: The Imam mistakenly said the Takbeer to stand up after the third rakat of Maghrib. The followers on the first floor stopped him, and he sat down. But we, on the second floor, didn’t know that, so we stood up until the Imam made salam. Then we sat down and concluded our prayer on our own. Is it okay?