Mediates the hydrolysis of diadenosine 5',5''-P(1)P(4) tetraphosphate (Ap(4)A), a signaling molecule involved in regulation of DNA replication and repair. H2O + P(1),P(4)-bis(5'-guanosyl) tetraphosphate = GMP + GTP + 2 H(+) Optimum pH is 8.0. Active from pH 6.8 to 9.5. No visible phenotype under normal growth conditions. Belongs to the Nudix hydrolase family.