Well, it's less about the DOT rating than it is the specific wet boiling point. The 5.1 standard has a higher minimum rating, but in practice a specific brand of DOT4 may exceed most DOT 5.1's, etc.
Silicone is completely incompatible with systems not designed for it and can't be mixed, but that's DOT5 and not relevant here.
DOT3, DOT4, and DOT5.1 are all polyethylene glycol (not silicone) and technically compatible, but DOT5.1 has different viscosity than DOT4, and most of the weekend racers are using high-end DOT4 fluids. Which DOT5.1 fluid did you use?
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