I joined salah in the masjid, I thought that the brother was praying Zuhr, after the salah ended he said that he was praying Sunnah. My intention was to pray 4 rakaats of Zuhr, is the salat valid?

According to the Hanafi Madhab, the Imaam and Muqtadi (follower) must be performing the same Fardh Salaat in order for the Salaat to be valid.

The Dhuhr Salaat you performed behind the brother who performed Sunnah Salaat is invalid. You should repeat your Dhuhr Salaat.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai