The Mark 4 utilized a near-identical nuclear explosive package to the 1561 assembly of the Mark 3 Fat Man, but the newly-formed Z Division of Los Alamos focused on re-engineering the electronics and conventional explosives packages to be more modular for easier assembly in the field.

[3] Many issues slowed and delayed the design and production of the Mark 4, the first of which was the formation and physical placement of Z Division facilities.

The early days of Z Division highlighted the lack of infrastructure and logistics of the nuclear weapon enterprise when production of weapon components had to slow due to lack of basic electrical supplies like soldering rosin and hook-up wire.

Along with other developments for the radar, the main component of the system was the APS/13 Tail Warning device which eventually became the 'Archie'.

Arming the Mark 4 required opening the casing's front hatch, removing the forward polar cap, two outer pentagonal lenses with their detonators, and two inner explosive blocks, and exposing the pit (the lenses and blocks weighed an aggregate 156 kilograms).

Beginning in 1950, the US military began to deploy non-nuclear Mark 4 assemblies overseas, including in the United Kingdom (July 1950), and aboard several aircraft carriers in 1950 and 1951.

A contingent of 9 Mark 4 non-nuclear assemblies were deployed to Guam in 1950, and Truman authorized the transfer of fissile cores to the base in April 1951.