[Daiminator]

Alright, it was the German Wikipadia-site...
On the english site ist sais: " DOT 5.1 can replace DOT 4 and 3, DOT 4 can replace DOT 3"

On the german site it sais: "Nicht alle Gummidichtungen in DOT-3-Bremssystemen sind auf DOT 4 ausgelegt, das heißt, sie können aufquellen, was zum Ausfall des Bremssystems führen kann."
Whitch means something like "Not all rubber-sealings in DOT 3 systems are constructed for the use with DOT 4. They might swell and cause a misfunction of the brake-system"

Its a pity that no literature is given, where the took that information form...