Highway 4's eastern terminus is at the interchange with Highway 19 at Qualicum Beach; however, the roadway continues north as Memorial Avenue for 4 km (2 mi) to Highway 19A. From Highway 19, it travels south for 3 km (2 mi) to a junction with an older Alberni Highway segment, now known as Highway 4A, at the small community of Hilliers in Coombs. Highway 4 veers directly west upon meeting this junction.

The majority of Highway 4's length is composed of a two-lane configuration. An inactive subdivision of the E&N Railway parallels Highway 4 all the way from the coast near Parksville to the city of Port Alberni.

West of Coombs Junction, Highway 4 travels for 11 km (7 mi) before meeting Cameron Lake. Highway 4 hugs the shore of Cameron Lake for 5 km (3 mi), then winds its way through MacMillan Provincial Park, which includes a tourist attraction known as Cathedral Grove. After Cameron Lake, Highway 4 climbs for 8 km (5 mi), following the Cameron River, to the Port Alberni Summit—the highest point on the highway (elevation: 411 metres (1,348 ft)), known by locals as "The Hump", at which it crosses from the Regional District of Nanaimo into the Alberni-Clayoquot Regional District—and then descends for 6 km (4 mi) to a spur that travels into the city centre of Port Alberni. Highway 4 enters the city limits of Port Alberni 2 km (1.2 mi) west of the main spur into the city, and 5 km (3 mi) later, exits Port Alberni via a bridge over the Somass River.

6 km (4 mi) west of the Somass River crossing, Highway 4 reaches the shore of Sproat Lake, following the lake shore west for 24 km (15 mi). Highway 4 then follows the Taylor River for 10 km (6 mi) to the summit of Sutton Pass (elevation: 240 metres (790 ft)), named after William John Sutton, and then turns toward the south-southwest to follow the Kennedy River. This winding bearing of Highway 4, which features very steep grades, continues for 23 km (14 mi) before reaching the southeast shore of Kennedy Lake. Highway 4 then follows the shore of Kennedy Lake southwest for 19 km (12 mi), and reaches a junction with a spur into the district municipality of Ucluelet (known locally as "The Junction") 5 km (3 mi) later, after which the route continues in a northwest direction. 1 km (0.6 mi) later, Highway 4 enters the Long Beach Unit of Pacific Rim National Park Reserve. Highway 4 goes through the national park for 22 km (14 mi) northwest before terminating 11 km (7 mi) later at Tofino.

From east to west:

Highway 4A is the previous alignment of Highway 4 east of Coombs. The '4A' designation was originally assigned in 1968 to the current section of Highway 4 between Coombs and Qualicum Beach. The 4 and 4A alignments east and north of Coombs, respectively, switched designations when the Parksville to Mud Bay section of the Inland Island Highway was completed in 1996. This older spur of Highway 4 terminates in the east at Parksville, 10 kilometres (6 mi) from Coombs.

Major intersections edit

The entire route is in Nanaimo Regional District.

• Official Numbered Routes in British Columbia by British Columbia Driving & Transportation