50 chevy 3100,

I can't answer your cornhead question, but I am pretty sure DOT 3 and DOT 4 brake fluids are compatible when mixed together, and those same fluids, if synthetic, will also mix with all regular brake fluid.

What you don't want is a DOT 5, which is a silicone product. It is not compatible with other fluids, and only recommended for a system designed for DOT 5 brake fluid.

Stuart