allDifferent4 :: Eq a => [a] -> Bool
allDifferent4 [] = True
allDifferent4 (x:[]) = True
allDifferent4 (x:xs) = notInList x xs && allDifferent4 xs

Das benötigt noch die Funktion:

notInList :: Eq a => a -> [a] -> Bool
notInList _ [] = True
notInList x (y:ys)
   | x == y = False
   | otherwise = notInList x ys