| Name: | M 4 |
| Type: | Submarine Monitor |
| Keel: | 08/12/1916 |
| Launched: | 28/07/1919 |
| Completed: | n/a |
| Builder: | Armstrong, Whitworth & Co |
| Yard: | High Walker |
| Yard Number: | 911 |
| Dimensions: | 1600disp surf, 1950disp subm, 290.0 x 24.7 x 15.75ft |
| Engines: | 2 x Oil engines, 2400bhp and 2 x Electric motors, 1600hp |
| Engines by: | Possibly by Vickers Ltd, Barrow |
| Propulsion: | 2 x Screws, 9.5knots |
| Construction: | Steel |
| Armament: | 1 x 12.0ins, 1 x 3.0ins AA guns; 4 x 12.0ins bow torpedoes |
| History: | |
| 30/11/1921 | Armstrong, Whitworth & Co, Newcastle |
| 30/11/1921 | Broken up |
| Comments: | 08/12/1916: Laid down for the British Royal Navy |
| Ordered as a K class submarine, to be named K21 | |
| 1917: Renamed M4 | |
| 26/11/1918: Order cancelled | |
| 28/07/1919: Launched to clear the slipway | |
| 30/11/1921: Sold to Armstrong, Whitworth & Co for breaking up |