Catalyzes specifically the hydrolysis of 4-guanidinobutanoate to 4-aminobutanoate and urea. Has no activity against arginine, agmatine, 3-guanidinopropionate and guanidinoacetate. 4-guanidinobutanoate + H2O = 4-aminobutanoate + urea Binds 2 manganese ions per subunit. kcat is 1012 sec(-1) (PubMed:12029055). kcat is 94.7 sec(-1) (PubMed:21600989). Optimum pH is 9.5. Homohexamer. Belongs to the arginase family. Agmatinase subfamily.