Backes, who sustained an upper-body injury when he was hit by Blackhawks defenseman Brent Seabrook during the third period of Game 2, was not a participant during the morning skate, and coach Ken Hitchcock did not disclose whether Backes would be in the lineup at that time.
The Blues captain did not take the ice for warm-ups.
Backes, who has one assist in two games in the series, was injured with 4:51 remaining in the game Saturday. He did not play in Game 3.
Seabrook will serve the second of a three-game suspension for the illegal hit Wednesday in Chicago.
St. Louis leads the best-of-7 series, 2-1. Game 5 is Friday in St. Louis (8 p.m. ET, CBC, RDS2, NBCSN, CSN-CH, FS-MW).