Is likely an oxygenase that introduces the hydroxyl group at carbon four of 2-octaprenyl-6-methoxyphenol resulting in the formation of 2-octaprenyl-6-methoxy-1,4-benzoquinol. Cofactor biosynthesis; ubiquinone biosynthesis. Component of the Ubi complex metabolon, which regroups five ubiquinone biosynthesis proteins (UbiE, UbiF, UbiG, UbiH and UbiI) and two accessory factors (UbiK and the lipid-binding protein UbiJ). Cells lacking this gene are photosensitive and are not able to grow with a nonfermentable source of carbon. They also fail to produce ubiquinone, and accumulate 2-octaprenylphenol and 2-octaprenyl-6-methoxyphenol. Belongs to the UbiH/COQ6 family.