The range of Vf for the LEDs is from 3.4V to 4V. Typically the Vf will be 3.4V but a 4V LED
is also within specification. Your Vout would be 4V.
On my 5940 LED driver board Loading...
I add a separate adjustable voltage regulator for the LEDs. This enables the Vout to be adjusted
for different Vf LEDs. The power dissipation is divided between the regulator and the 5940.
The regulator can dissipate 1W at 25degC without a heatsink.
The schematic is in the datasheet.
(* jcl *)