Is PROLOG is an example of a fourth generation programming language?

Not really. Officially, there is no such thing as a fourth-generation language (4GL). The term 3GL first appeared after the third-generation of hardware first appeared and applied to all high-level languages, including PROLOG. Today we do not use terms such as 4GL or 5GL as they are just meaningless buzzWords coined by marketing types. Today, all high-level languages are classified according to whether they are imperative, declarative, functional, object-oriented, and so on. PROLOG, in its un-extended form, is an example of a declarative language.