A compound represented by the following formula (I), a salt, or a solvate thereof: wherein, R1 and R2 are the same as or different from each other and each represents a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, a cycloalkyl group, a cycloalkenyl group, a non-aromatic heterocyclic group, an aromatic hydrocarbon cyclic group, an aromatic heterocyclic group, an acyl group or an alkylsulfonyl group all of which may be substituted and wherein one, or both, of R1 or R2 is an aromatic, or non-aromatic, heterocyclic group which may be substituted; R3 represents a hydrogen atom, a halogen atom, a cyano group, an alkyl group, an alkenyl group, an alkynyl group, an aromatic hydrocarbon cyclic group, an aromatic heterocyclic group, a nitrogen atom, an oxygen atom or a sulfur atom all of which may be substituted if possible; and R4 is a pyridyl group which may be substituted.