* Saudi becomes world's fourth largest oil refiner * Plans to reach 8 mln bpd of refining would surpass Exxon * refining and trading could shift Kingdom's OPEC calculus By Libby George and Ron Bousso LONDON, May 22 (Reuters) - Saudi Arabia's rapid transition into one of the world's largest oil refiners adds an extra dimension to the oil exporter's role as the driver of OPEC policy. When it attends OPEC's next meeting in two weeks, it does so with major new state-of-the-art oil refineries that can profit from cheaper crude and reviving world fuel demand - exactly as international oil firms have over the past six months. The kingdom **w has stakes in more than 5 million barrels per day (bpd) of refining capacity, at home and abroard, landing it a place among the global leaders in making oil products.