DOT 3 & 4 are poly-glycol, which is completely soluable in water. (That's how they get wet boiling point that's higher than 212F.) Difference is that DOT 4 requires higher boiling points.

DOT 5 is silicone, that's actually really bad for Honda brakes, unless you change lots of rubber parts & flush it out regularly. It doesn't absorb water, so if ANY water gets in, the boiling point suddenly drops to 212F.

DOT 5.1 has the high boiling points of DOT 5, but it's a poly-glycol fluid like DOT 3 & 4. Often a DOT 5.1 fluid is labeled as DOT 4 because it satisfies that spec. also.