The KL4 is purely a regulated boost circuit. So Vin above the Vf of the LED puts it in to direct drive. This is usually around 7V. Try not to go any higher as nothing will protect the LED from being overdriven.
For the bottom end of the operating range, it will stay in regulation to approximately 3V, but keeps running down to 2.4V. Below that, it may continue to glow.