Freddie Ladapo scored a stunning solo goal to temporarily reduce the deficit.
However, just a minute later Dan Gosling fired home to grab a fourth for the visitors.
Defeat for Paul Warne and his side means they are three points from safety but still have three games in hand on Birmingham City.
Story of the match
Rotherham made four changes from the side that beat Sheffield Wednesday 2-1 a fortnight ago. Jamal Blackman, Shaun MacDonald, Michael Ihiekwe and Richard Wood all came in for Viktor Johansson, Wes Harding, Florian Jozefzoon and Lewis Wing.
Watford on the other hand were unchanged from their important 2-1 victory in Cardiff City three days ago.
The first decent chance of the game fell to the home side after a good cross from Daniel Barlaser which caused panic in the Watford 18-year box before the ball was cleared as far as Wood who fired his effort wide of the target.
Despite the Millers early chances, it was the Hornets who took the lead after a great floated ball into the box by Philip Zinckernagel was headed home by Sierralta.
Rotherham reacted well to going a goal behind and nearly levelled the game after Angus MacDonald glanced just wide from a Barlasrer set-piece.
The visitors first real chance after going ahead came in the 25th minute, with good work from Joao Pedro before he set Kiko Femenia free to whip in a good ball but great work from Matthew Olosunde saw the defender avert the danger.
Just a minute later, Watford doubled their lead after a good effort from Nathaniel Chalobah was parried by goalkeeper Nick Blackman into the path of Sarr who fired home with left-foot.
Watford nearly put the game out of sight when Sema crossed for Pedro but his effort was blocked by Olosunde once again. The resulting clearance found its way to Chalobah who fired through a crowd of bodies before his effort was expertly saved by Blackman.
Watford ended the game as a contest on the 40-minute mark with Sema grabbed the Hornets third. The former Udinese man picked the ball up on the edge of the box before he flicked the ball up and drilled his volley into the back of the net.
With the game in the bag, Xisco Munoz substituted Sarr and Femenia and replaced them with Andre Gray and Jeremy Ngakia.
Rotherham were awarded a controversial penalty after William Troost-Ekong was adjudged to have brought down Michael Smith. Smith dusted himself down to take the spot-kick, but his penalty was well saved by Daniel Bachmann.
Blackman prevented the scoreline from being a cricket score with another brilliant save after a good flick from Gray found Pedro but the 'keeper got down low to deny the Brazilian.
Rotherham scored a consolation goal in the 68th minute when Ladapo fired struck from distance into the top right-hand corner giving Bachmann no chance.
Immediately from kick-off, Watford regained their three-goal advantage after Blackman saved from Pedro once again but the ball fell kindly to substitute Gosling who grabbed the Hornets fourth.
Neither side had any real clear cut chances in the final 15 minutes with Watford breezing to a comfortable victory.
Man of the match
The Danish midfielder grabbed himself another two assists in another fine individual performance. He has been one of the players who has benefitted from the appointment of Munoz and started to nail down his place in the middle of the park.
His pace, dribbling and quality of passing were all on display tonight and his free transfer in January is starting to look like a very good acquisition for the Hornets.