HMS L4 was a L-class submarine built for the Royal Navy during World War I.

The L-class boats were enlarged and improved versions of the preceding E class.

[1] For surface running, the boats were powered by two 12-cylinder Vickers[2] 1,200-brake-horsepower (895 kW) diesel engines, each driving one propeller shaft.

[3] On the surface, the L class had a range of 3,200 nautical miles (5,900 km; 3,700 mi) at 10 knots (19 km/h; 12 mph).

On 20 October 1927 off Hong Kong, L4 and HMS L5 rescued crew of the merchant ship SS Irene from a pirate attack after firing her deck gun.