First-half goals from returning full-back Kieran Gibbs and Matheus Pereira set Albion on course to stretch their unbeaten run to eight games - and their goal tally to 35 in 18 matches.

Famara Diedhiou's left-foot finish on 80 minutes briefly gave City hope, but the Baggies took just two minutes to restore their two-goal cushion through Hal Robson-Kanu.

And Matt Phillips added another on 87 minutes as Albion hit four goals for the third time in six home matches for their fourth straight victory.

Slaven Bilic's Baggies now lead second-placed Leeds United by two points again, but have opened up a seven-point gap on the play-off pack, while Lee Johnson's City remain seventh, still just a point outside the top six.

Albion boss Bilic rang the changes as he brought back midfielders Jake Livermore and Romaine Sawyers, and defender Nathan Ferguson after suspension, while also picking fit-again Gibbs for his first start since 24 August.

And the former Arsenal and England full-back had a dream return when he scored on 10 minutes - his first goal since April, making him the 14th different Albion player to score this season for the Championship's top scorers.

The influential Grady Diangana got away with a foul in the build-up but there was no doubt about the quality of the final ball when he instinctively backheeled from close to the byeline to bamboozle the Bristol defence for the on-rushing Gibbs, who was left free to drill home from eight yards.

Diangana was then involved in Albion's second goal too, being fouled by Bailey Wright on edge of box - and Pereira exacted full punishment.

With almost breathtaking deftness, the on-loan Brazilian floated a beautiful left-foot curling free-kick over the wall and into the top right corner, to leave flat-footed City keeper Daniel Bentley helpless.

City, who had Korey Smith making his first start since 19 April after seven months out with a broken foot, remained a threat.

Ashley Williams had a goal disallowed for offside before Diedhiou finally clawed one back. But Albion finished strongly to confirm the Robins' first defeat in six games.

In-form Wales international Robson-Kanu hit his fifth goal in seven league games for the Baggies before Phillips rounded things off with his seventh of the season.

Albion head coach Slaven Bilic told BBC WM:

"We played really good. To win 4-1 against a side like Bristol City, it had to be a good performance. They are a very solid team. But we made them not solid enough.

"I was very proud of the way we managed the game. We showed resilience and quality without the ball, but to attack with the numbers we do give more players the chance to score goals.

"The only negative is that we should have scored more goals. But playing like this will give us the chance to win many more games.

"We showed physicality, shape, one against ones, pace, skill, crosses, second balls - everything. We slaughtered them on second balls and that's what pleased me."

Bristol City boss Lee Johnson:

"We came with ambition but we didn't start. We were shy on the second balls and we weren't aggressive enough. I apologise to our fans because they don't deserve that.

"Disappointing is an understatement. One thing you have to do at a place like this is earn the right to play. But I take full responsibility - it's my team, my identity, and we didn't do enough.

"I don't think it was a lack of effort but we were found wanting in too many areas against an albeit good side. We're hurting after a loss and we've taken a bit of a pride bashing.

"In one sense, West Brom looked like a Premier League outfit, which was a good test for us, but we should stand up to that better than we did."