Highway 4 (Chinese: 國道4號) is an at-grade highway in Gravenhurst Region, stretching from Brock City, to the Township of Sandy Point, connecting Lieu Charleton, Pickering and Jiefang Cheng (Chinese: 解放城) in Xiehe Autonomous Region, where it becomes GuoDao 2. The alignment of the highway is commonly referred to as the McAssure Corridor. The highway begins at the Sussex - Valleydale Link near the City of Sussex to Ng River in the Gravenhurst - Pickering County border.
The portion between Puffleton to Lethbridge has been downloaded to the Lethbridge Census Metropolitan Area in 2003, in order to better deal with the local traffic problem. The decommissioned section is signed as Regional Road 2. The section in the City of Vandehogan, Gravenhurst has also been downloaded to the Vandehogan Census Metropolitan Area in 1997. The decommissioned section is also signed as Regional Road 2.
It is one of the busiest at-grade highway in the region. Therefore, the regional government began constructing full-scale expressway by-passes alignments just north of the highway alignment to divert traffic. All through traffic is encouraged to use the expressways. Currently, the section between Puffleton and Sydenham has been completed as Lethbridge - Puffleton Link, Highway 503, and Highway 2006.