Driver for higher forward voltage?

What is the best way to deal with a forward voltage above 3.7 V in single cell flashlights?
UV or green LEDs can have up to 4.3 V at high currents.

You will need some sort of boost-type driver since these leds. However at low drive current the voltage of the led may be close to battery voltage or even lower, so the ideal solution is a buck-boost driver, but I don’t know any good one on market yet…