Catalysis of the reaction: (R)-2-(carboxymethyl)-5-oxo-2,5-dihydro-2-furoate + H+ = 5-oxo-4,5-dihydro-2-furylacetate + CO2.
4-carboxymuconolactone carboxy-lyase (4,5-dihydro-5-oxofuran-2-acetate-forming)