Asymmetrically hydrolyzes Ap4A to yield AMP and ATP. H2O + P(1),P(4)-bis(5'-adenosyl) tetraphosphate = AMP + ATP + 2 H(+) Divalent metal ions. Mg(2+), Co(2+), Mn(2+), Zn(2+) or Ca(2+). Monomer. Mutagenesis suggests that interactions with P1- and P4-phosphate are minimum indicating the enzyme may have a wide substrate range. Belongs to the Nudix hydrolase family.