The type 4 engine is the name given to the 1700cc, 1800cc and 2000cc engines from late Bay Window buses and early T3 buses, the T411 and T412, and the Porsche 914.
This engine was similar in style to the Type 1 engine, but included several improvements, such as a spin-on oil filter, and through case bolts. It also had a completely different 'pancake' style cooling system.
This engine is sometimes transplanted into other vehicles such as the Beetle. See Type 4 conversion.
Much more information is available here:
Also try these forums:
www.shoptalkforums.com/viewforum.php?f=1
www.shoptalkforums.com/viewforum.php?f=47
See also Type 1 engine.