Given two Coxeter's frieze patterns with the same width and consisting of positive numbers, choose a row and consider the periodic sequence of the differences of the respective entries of the two friezes. We ask for which rows this sequence must change sign at least four times over the period. We prove that this is the case for the first and for the second non-trivial rows, and that this is true, for all rows, for an infinitesimal version of the question. The article also contains expository material on the four vertex theorem and on Coxeter's frieze patterns.