The Extension No. 4 mine, owned by Canadian Collieries (Dunsmuir) Ltd., opened up in 1910, closed in 1913 then reopened that same year and operated until 1917. The mine is part of the Extension Colliery which consists of four mines, all on the Wellington seam within the Extension basin. The Extension No. 1, No. 2 and No. 3 mines (092GSW028), are situated on the southwest limb of the Extension anticline and the Extension No. 4 in the northeast limb.

The seam worked in the No. 4 mine is of high volatile bituminous rank and occurs toward the base (Early Campanian Northfield Member) of the Upper Cretaceous Nanaimo Group, Extension Formation. The seam is interbedded with sandstone, shale and conglomerate. The floor is generally sandstone and the roof varies from shale to conglomerate. Refer to the Bebans mine (092GSW026) for a description of the Wellington Seam in the Nanaimo Coalfield.