Catalyzes the hydrolysis of 4-guanidinobutanoate to gamma-aminobutyrate (GABA) and urea. Involved in an alternative, arginase-independent arginine degradation pathway via GABA. 4-guanidinobutanoate + H2O = 4-aminobutanoate + urea Binds 2 manganese ions per subunit. Amino-acid degradation; L-arginine degradation. Belongs to the arginase family. Agmatinase subfamily.