Given the rising societal pressure towards sustainable waste management and resource efficiency, in a more circular economy, an increased use and diversification of supplementary cementitious materials (SCM) will be necessary to achieve the CO₂ mitigation goals. The current study addresses the development of self-compacting concrete, replacing part of the cement (the primary source of CO₂ emissions) by metakaolin and wastes derived from two industrial sectors operating in the “Galicia–North of Portugal Euroregion”: wood manufacturing and natural stone quarrying. A study was carried out at the mortar level to investigate the effect of the mix design variables on several engineering properties of the self-compacting concrete. Statistically designed experiments reveal that an increase in water/powder volume ratio has a dominant effect on the fresh state properties, whereas the water/cement weight ratio has a dominant effect on the hardened state properties. A like-for-like comparison of the proposed quaternary blends and previously studied binary/ternary blends indicates that these mixtures exhibit improved self-compacting ability, greater compressive strength, and can offer interesting opportunities to reduce the unit cost and environmental impact of self-compacting concrete per m³. Four different mortar mixtures were optimised to achieve excellent self-compacting ability yet with distinct compressive strength levels at 28 days (65, 70, 75, and 80 MPa). A single measure of the material efficiency is proposed herein to reflect the engineering properties improvement (workability, compressive strength, and durability) over its economic (unit cost) and environmental impact.