Ideas about MK IV combination from several sources:

Weight compenation from MK III,
but bicycle pump upside down and with a hollow shaft.
Air buffer for each leg.

V_belt drive from Roland: or
old timing (teeth) belt from car engine (renieuwed after xx KM and thrown away)

Linear actuator from Ian:

Direct_drive motor (from a Electric bike?) (Hub motor) (no speed reduction?)

Try to make a design that can be mede with simpel tools.
Anny comments are welcome.
 
But first priority is completing MK III.

Premature principle hand drawing with speed reduction.                                     example of teeth belt use see below,. mkIV SketchUp
 

Collection timing belts, Alu tube and staf, Steel tube,
Staf Ertalon LFX oil for piston and slide bearings,
Piston seals.