P-4 

In the mid-50s, the USAF decided to implement a lower impedance communication system. P-helmets as the P-3 with modification implemented, changed designation to “P-4 Flying Helmet”.

Unlike the old system which consisted of sponge rubber half-rings between ear pad and helmet shell, the new earphone assemblies were fitted on spring provided mounts called, because of their shape  , “Top hats”, tied to short nylon strings which protruded through the helmet shell to be secured over a phenolic resin stud.

The most visible change implemented to P-4 helmets was the visor yoke that gave the pilot better upwards visibility and carried less risk of snagging between helmet and parachute risers. The J-shaped up lock arms on at each end were longer, allowing the visor to move completely out of the pilot's field-of-view, additionally the visor yoke no longer had the square handle in the middle.