The cooperation sustained on the same site over the years presented a completely different engineering equation with the raising of the waste storage facility and the commissioning of new cells within the scope of the fourth phase. Field data obtained from previous phases made it possible to reflect the base profile and slope stability parameters directly into the design; this ensured that the panel cutting plan and welding paths were optimized directly according to the site geometry rather than in the office environment.

The prominent technical element in this phase was the management of connection zones between the liner under active operation and the newly constructed cells. Additional manufacturing carried out while active storage continued required a staged capping plan according to the waste flow direction; at each stage closure, dual-track air testing was applied to wedge welds as well as extrusion welds, and connection details were processed into as-built drawings.

Due to the nature of the West African supply chain, roll planning was kept synchronized with port deliveries and the on-site internal transportation schedule; a stock protocol was implemented in on-site temporary storage areas to protect rolls against UV and mechanical damage. This discipline was the fundamental factor behind the completion of the fourth phase without operational interruption.