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OK, try to catch more specimens of both variant and then we could know more. I do not know Cantharis quadripunctata with uniformly red pronotum and only apical spot on head. Maybe it should be extremely dark variant of Ancistronycha cyanipennis (= violacea). I am almost sure you have two species on your pictures. You could send them to the local specialist, but, believe me, please, I have big experiances in Cantharidae.
Libor