Designed and entered service in the early 80's, the Mk4 series of aircrew helmets were a big departure from the design of the previous Mk2 and Mk3 helmets.
The main fundamental difference was abandoning the acoustic tube based headset of the Mk2 and Mk3 helmets, and going back to a traditional two-speaker headset. This change meant the helmet shell had to be re-designed to allow space for the deeper ear-pieces, so the Mk4 shell has bulbous lobes on each side, when compared to the flatter Mk2 and Mk3 shells. Speaking to ex-RAF aircrew, I have ascertained that the Mk2 / Mk3 headset sound was not popular. The acoustic tubes deliver a distant and echoey sound when compared to helmets with one speaker per ear, such as all helmets previous to the Mk2.
Not many features of the Mk2 and Mk3 helmets remained, but notably the re-design brought about a new internal webbing harness, simplified oxygen mask-hook attachment, and a new way of tensioning the ear-pieces to the head.
The visors themselves shared the same optical pieces as the Mk3C, as did components of the visor pivots, but the visor arms and latch were altered, including moving the latch from the right to the left hand side.