In Lethbridge, the highway turns north onto 43 Street as a major arterial road with a speed limit of . It intersects the major east-west arterial routes of 20 Avenue and South Parkside Drive in south Lethbridge before crossing the Canadian Pacific Railway tracks a final time, then ends at an intersection with Crowsnest Trail. With only four traffic lights, it provides an alternative to the busy Mayor Magrath Drive, the main north-south route in Lethbridge, which has a dozen sets of lights over the same distance. The section of 43 Street near 6 Avenue South is the busiest part of Highway 4, carrying over 19,000 vehicles per day in 2017. Between Lethbridge and Wilson Siding, traffic quickly decreases to less than 10,000 vehicles per day, and less than 5,000 vehicles per day for the remaining until Coutts.