The LEDs voltage tolerances are often more than 0.1V either way, so what voltage do you aim at...?
Well fungus, I would say that I would add the 0.1V to the 4V maximum forward voltage so 5V - 4.1V = 0.9V drop voltage, then to calculate the resistor needed (if I use a resistor vs a chip) it would be 0.9V / .02mA = a 45Ω resistor (or the next larger value), HOWEVER
since I am always wrong about these things my gut is telling me to SUBTRACT 0.1V from the 4.0V BECAUSE it is the MAXIMUM forward voltage and if it is maximum then I can't go beyond it right? So 5V - 3.9V = 1.1V and 1.1V / .02mA = a 195Ω
resistor (or the next larger value). I am aware that you may have been asking a rhetorical question, but I wasn't sure.