Interstate 15 becomes Highway 4 as it crosses the international border between Montana and Alberta into Coutts, the only border crossing between Montana and Alberta that is open 24 hours year-round. The highway curves north out of Coutts alongside the railway to a three-way intersection with Highway 501 which leads west to Del Bonita. The two highways continue concurrently to the north, briefly paralleling Milk River. The concurrency ends and Highway 501 splits to the east into the community of Milk River, becoming Railway Street, while Highway 4 continues north across the river to bypass the town.