CMB 4 was long and 8 ft 6 inches in the beam. She displaced 5 tons drawing 2 ft 9 inches of water. Power was provided by a 1 shaft Thornycroft V-12 petrol engine which developed 275 bhp and achieved a top speed of 24.8 knots. The boat was armed with one 18-inch torpedo and four .303 Lewis machine guns. It carried a crew of 3.
In May 1916, Lt. W.N.T. Beckett MVO DSC took command of the newly built HM Coastal Motor Boat 4. In December 1916 he proceeded to Dunkirk in charge of the 3rd CMB Division and operated on the Belgian coast. Beckett was in command of a Divisional CMB attack on German destroyers at Zeebrugge on 7 April 1917; as a result one was sunk and one very seriously damaged. For these actions Beckett was mentioned in Despatches and was awarded the Distinguished Service Cross (DSC).
The boat, under the command of Lt. Augustus Agar, V.C., was made famous by his daring exploits in the Baltic Sea against the Bolsheviks in 1919 where she operated with her sister ships in activities such as the raid on Kronstadt.
CMB 4 is preserved at the Imperial War Museum Duxford.