HMS B4 was one of 11 B-class submarines built for the Royal Navy in the first decade of the 20th century.

[1] For surface running, the boats were powered by a single 16-cylinder 600-brake-horsepower (447 kW) Vickers petrol engine that drove one propeller shaft.

[1] On the surface, the B class had a range of 1,000 nautical miles (1,900 km; 1,200 mi) at 8.7 knots (16.1 km/h; 10.0 mph).

They could carry a pair of reload torpedoes, but generally did not as they would have to remove an equal weight of fuel in compensation.

[4] B4 collided with a dredger when entering Portsmouth harbour on 21 July 1906 and was badly damaged as a result, having to be beached before being taken into dry dock for repair.