(provided that in the formula (a4), Rb and R2 are as defined in the formula (a3), respectively, R3 is a C2-10 alkylene group or a C4-10 bivalent alicyclic group, R4 is a hydrogen atom or ???NHZ1Z2Z3, wherein each of Z1, Z2 and Z3 which are independent of one another, is a hydrogen atom, a C1-4 alkyl group or a C1-6 hydroxyalkyl group, provided that at least a part of R4 is required to be ???NHZ1Z2