Katz argues, correctly I believe, that realistically we cannot give full attention to all four - thus prioritization is required. He analyzes the four and concludes:
Despite its growing assertiveness of late, Beijing’s behavior has been less aggressive than that of Moscow since its annexation of Crimea, support for the rebels in eastern Ukraine, and increasing threats to European stability.Publicly available information about U.S. actions overseas suggests the government has essentially drawn these same conclusions. Hat tip to RealClearDefense for the link.
While some are doubtful that Iran truly seeks rapprochement with the West, there is little question that ISIS, Al Qaeda, and similar groups are not open to diplomacy, and that they would overthrow all those Middle Eastern governments which fear Iran (as well as topple the Islamic Republic itself) if they could.
Analysis suggests, then, that at present, America should prioritize Russia and Sunni jihadists over China and Iran.