I believe you may be able to cite to Guderian's Panzer Leader for this claim. If I recall correctly, he stated that the Somua was superior in firepower and armor to the Panzer III (and probably the IV too, since it was still armed with the L/24 infantry support gun), but it was ineffective against the Panzer III due to German use of radios, crew configuration, and the fact that the French dispersed their armor with the infantry rather than using concentrated armor as the Germans did. Also, see the Panzer IV page, which cites to other sources supporting this claim.