Welcome to the new Wizard Outboard Forum.
WD4 is 1946 and is 6HP made by Mercury. Gas to oil mixture is 16:1 for practical purposes. It has a hard to find water pump impeller that you can get if you search the old posts on this site.
Try Ferguson-Poole for impellers. http://www.fergusonpoolemarine.com/enginecoolingsystem.html