Erling Haaland scored his 20th goal for Manchester City as he helped his team move to the top of the league.
With struggling Saints the latest side to find out how hard it is to keep a player quiet, the 22-year-old Norway forward has taken only 13 games to reach that mark.
Haaland was close to giving them an early lead when he hit the inside of the post after clearing the ball.
Joao Cancelo ran on to Phil Foden's pass and cut past James Ward-Prowse before finding the bottom corner of the net.
Foden got on the score sheet himself after Kevin de Bruyne found him in the box.
That put City in complete control at the break and they wrapped up the points at the start of the second half when the Algerian forward scored.
The only thing missing for the home fans was another Haaland goal.
Haaland was unable to get on the end of De Bruyne's cross under pressure from Mohammed Salisu, and then brilliantly denied by Bazunu, who dove at his feet as he charged through goal.
As the chances kept coming, you just knew Haaland would take one, and so he did, meeting Cancelo's cutback with a typically precise low finish to make it four.
Even though they only managed one touch in the City area, they never looked like avoiding a fourth straight defeat.
The last time City was at the top was on August 20th and they might not be there for long.
Regardless of their current position in the table, it's clear that they're in a good place.
The way in which the defending champion are despatching opponents is the most ominous sign of how well they are doing.
Since the start of the season, City have scored 33 goals in nine league games, 13 more than their nearest rivals.
Haaland has 15 league goals for the season, matching the tally of De Bruyne when he finished as the top scorer in that competition in 2021, but his team-mates keep scoring as well.
The performance was more important to the manager than the outcome.
St Mary's did not play like a team that has given up on their manager, despite reports suggesting that Ralph Hasenhuttl is on the verge of losing his job.
Their main aim was to limit damage, but they have not been able to prevent worse losses at the stadium.
When City came flying out of the blocks, Hasenhuttl's side defended well and continued to do so even after the game had ended.