Q: Salaah-190: As the Imaam started the Witr salaah, the Musalli followed Him thinking that it was the Taraaweeh salaah. Will the Musalli's Witr salaah be valid?

Answer: In the stated case, the Musalli should stand up after the Imaam Makes Salaam and add a fourth Rakaah to make his salaah one of four Rakaahs. This four Rakaahs will then be regarded as Nafl salaah and he will still have to perform the Witr salaah.

Fatawa Rahimiyyah vol.2