Mill No. 4 was built in 1892, just 300 yards East of Mill No. 3 on the shore of the Ottawa River. Like Mill No. 3, it was steam-operated and equipped with the latest technology of its day. Mill No. 4 had a a 125-foot high "Iron Bottle" burner. It was the last mill built by the McLachlin Bros. Due to an economic downturn in the 1920s, only Mills 3 and 4 were still in operation by 1923. They ceased operation in 1929, and Mill No. 4 was finally demolished in 1936.
Caption reads "General View of No. 4 Showing the lumber conveyor and a glimpse of the end of No. 4 sorting table. The motor houses in this case are hung under the trestle. the tracks in the foreground leads to the gravel pit where ballast is obtained for the thirty miles of track in the lumber yard."
Caption reads "Another View of No. 3 Mill Looking down the lumber conveyor from sorting table. The seven cabins house the electric motors that drive the conveyor. Each drives twenty nine rolss. Lumber moving at the rate of three and a half feet a second takes nearly five minutes to reach the sorting table."
Caption reads "All the edgings, the slabs too small to be re-sawn, and the waste from the re-saw fall on a transfer and pass under the five slash-saws shown here. These saws, electronically driven, cut this material into four-foot lengths and deliver it to the main conveyor running full length of the lath mill."
Gillies Bros. new 'fireproof' mill at Braeside was built in 1920 with reinforced concrete and eight inch brick walls on solid rock. The mill had a cutting capacity of 150,000 feet daily when it opened in 1921. The mill was expanded and modernized in 1969 and portions of its storage and kiln operations were rebuilt after a fire in 1976. The buildings were completely demolished in 2012.