What is the difference between Dot3 and Dot4 fluid? Can Dot4 be used without flushing system?

Mark


1946 1/2 ton - 1962 "261"
1953 1/2 ton 5-window - "235"
1955 1/2 ton - 55 "265" Corvette motor
1959 1/2 ton Fleetside - "283"