There are Chinese DC CC CV switching mode buck regulators on ebay. I'd use one of these for higher efficiency and less heat. For 4 diodes x 4.5v you'll need to tune it for at least 18v and the current limiting should also be tuned and verified before you connect it to the diodes. You'll need at least 20v battery to supply it unless you decide to provide each diode with it's own. And the battery must have high current capability. Also i'd replace all electrolytic caps with better brand ideally, Japanese Rubycon.