The 26-year-old delivered a flawless performance at the shooting range, hitting all 20 targets and finishing less than 12 seconds behind Sweden's Elvira Oeberg, who took third place despite missing three targets.
France's Lou Jeanmonnot secured the win with a clean shooting display and a time of 35:52.3, finishing 1 minute 8.2 seconds ahead of Sidorowicz.
Sweden’s Ella Halvarsson claimed second place with one missed target.
Sidorowicz's previous World Cup best was 22nd in the individual race in Anterselva, Italy, last season.
Her fourth-place finish marks the best result for a Polish biathlete since Monika Hojnisz-Staręga also placed fourth in the women’s 12.5 km mass start race in Pokljuka, Slovenia, in January 2020.
Anna Mąka, the second-fastest Polish competitor, finished 22nd with one missed target.
Hojnisz-Staręga, a 2013 world championship bronze medallist and one of Poland's top biathletes, is still regaining her form after a two-year maternity leave.
The 33-year-old plans to return to international competition early next year, aiming for the world championships in Lenzerheide, Switzerland, in February, according to the Polish team's Norwegian coach Tobias Torgersen.
Natalia Sidorowicz. Photo: PAP/EPA/MARTIN DIVISEK
Monika Hojnisz-Staręga. Photo: PAP/Grzegorz Momot
Last season, the Polish team ranked 10th in the Nations Cup standings.
This World Cup season began on November 30 and includes a series of competitions in Kontiolahti running through December 8.
Biathlon is an Olympic winter sport that combines cross-country skiing and rifle shooting.
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Source: PAP