TOKYO, Dec 15 (Reuters) - Japanese government bond prices were steady to firmer on Tuesday, with the benchmark 10-year yield hitting a four-month low, as investors bet central banks in Japan and the United States would keep their loose monetary policies this week. The U.S. Federal Reserve could strengthen its forward policy guidance while the Bank of Japan is expected to extend some of its pandemic relief measures such as corporate bond buying. Benchmark 10-year JGB futures rose 0.04 point to 152.16, though trading was slow with a thin volume of 13,120 lots. The 10-year JGB yield fell 1 basis point to 0.000%, a level last seen in early August. The 20-year JGB yield fell 0.5 basis point to 0.380% while the 30-year yield fell 0.5 basis point to 0.620%. At the shorter end, the two-year JGB yield was flat at minus 0.145%, while the five-year yield was unchanged at minus 0.130%. (Reporting by Tokyo Markets Team; Editing by Subhranshu Sahu)