Catalyzes the decarboxylation of four acetate groups of uroporphyrinogen-III to yield coproporphyrinogen-III. 4 H(+) + uroporphyrinogen III = 4 CO2 + coproporphyrinogen III Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 4/4. Homodimer. Belongs to the uroporphyrinogen decarboxylase family.