HD-4 or Hydrodome number 4 was an early research hydrofoil watercraft developed by the scientist Alexander Graham Bell.

Baldwin studied the work of the Italian inventor Enrico Forlanini and began testing models based on his designs, which led them to the development of hydrofoil watercraft.

On returning to Bell's large laboratory and boatyard at his Beinn Bhreagh estate near Baddeck, Nova Scotia, they experimented with a number of designs, culminating in HD-4.

Bell and associate Frederick W. "Casey" Baldwin began hydrofoil experimentation in the summer of 1908, as a possible aid to airplane takeoff from water.

On September 9, 1919, on the Bras d'Or Lakes, at Baddeck, HD-4 set a world marine speed record of 70.86 miles per hour (114.04 km/h), that stood for almost a year until being beaten by Gar Wood in Miss America.