The MB-4 was designed to meet a June 1919 specification from the United States Post Office Department for a two- or three-engined mailplane, required to carry 1,500 lb (682 kg) of mail on a single engine.

[3] It had a conventional tailwheel undercarriage and had two separate tail assemblies, which were standard MB-3 empennages.

[1][4] It was described as the "worst thing on wings" by Jerome Fried, the general plant superintendent of Thomas-Morse.

[4] At least one MB.4[1] (and possibly 3[4]) was delivered to the United States Army Air Service where it was stored before being scrapped.

[1] The MB-4 was a failure, having extremely poor flying characteristics and being described as the "worst thing on wings",[1] and saw no service other than the trials by the manufacturer, US Army and the US Post Office.