wherein at least one of R1, R2 and R3 is an alkoxy group or a halogen atom, the other(s) being selected from the group consisting of an alkyl group, an alkenyl group and a hydrogen atom; R4 is an alkylene group part of hydrogen atoms of which may be substituted with a fluorine atom or an alkyl group, a phenylene group part of hydrogen atoms of which may be substituted with a fluorine atom or an al