Under this agreement, the Basque Government will cover 55% of the cost of the new infrastructure, while the Bizkaia Provincial Council will assume the remaining 45%. Metro Line 4 is designed to connect Bilbao's southern neighborhoods with the rest of the railway network, promoting sustainable mobility.

In addition to the funding distribution, the agreement also foresees the necessary inter-institutional contacts to promote the transfer of the current Enkarterri railway line. Although the train service is managed by the Basque Government, the infrastructure remains under the control of the Spanish Government, through Adif. The ultimate goal is for it to be transferred to Lakua so that Line 4 can utilize this network and extend to Enkarterri.

With a length of 6.7 kilometers, the new line will feature six new stations. From Matiko, it will extend underground towards the university area in the Deusto neighborhood, connecting with the Euskadi Square area and Elíptica Square, where it will link with lines 1 and 2. Subsequently, it will proceed towards Basurto through Bilbao's upper neighborhoods, Irala, and Rekalde. If all goes as planned, construction in Rekalde is scheduled to begin by the end of 2027.

A spokesperson for the Ministry of Sustainable Mobility stated that Line 4, along with other major projects underway this legislature, is "key for the coming decades in Bizkaia's mobility" and "will undoubtedly allow this historical territory to become a benchmark for sustainable mobility." Euskal Trenbide Sarea (ETS), the public entity under the Department of Sustainable Mobility, is already making parallel progress with the necessary studies and projects prior to the tender for the works, aiming to "accelerate" the projects and commence construction before the end of 2027.