Largest family without A∪B⊆C∩D

Let F be a family of subsets of an n-element set not containing four distinct members such that A∪B⊆C∩D. It is proved that the maximum size of F under this condition is equal to the sum of the two largest binomial coefficients of order n. The maximum families are also characterized. A LYM-type inequality for such families is given, too.