I think it's primarily their heat-handling capabilities. The DOT4 will have both a higher wet and dry boiling point. They are compatable, whereas a DOT5 is silicone-based and is another animal altogether.

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"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo