Koby Jennings' potential was on full display as the young jockey rode a remarkable four winners on a heavy track at Rosehill.

The 21-year-old apprentice who came to Sydney from Victoria via Darwin and Canberra, steered Ravi, King's Officer, Kellyville Flyer and the Chris Waller-trained So Willie to victories on Saturday.

Sydney's big wet memorable for the apprentice as the best day of his riding career so far.

For Jennings it was the first time he rode a treble, let alone four winners in a day, taking him to 16 metropolitan wins for the season.

The rangy youngster, who graduates from his apprenticeship in four months, said his goal heading to the metropolitan meeting was to "get through" on a heavy track with a rare book of eight rides for the day.

Getting the wins was a bonus, he said.

"You come here with all this rain and a wet track and you hope that they get through a big book of rides," Jennings said.

Winning his first race for premier Sydney trainer Waller was another box ticked, while an injury to Winona Costin gave him the late pick-up ride on Ravi.

"It's my first winner for Mr Waller and that's been a goal so that's another box ticked," Jennings said.

The wins take Jennings to equal second on the NSW metropolitan apprentices' premiership for 2015-2016, equal with Costin who won the title last year.

Rory Hutchings (29 winners) has what appears to be an unassailable lead with less than two months remaining.

Costin was injured in a trackwork accident at Randwick on Friday.