If the goal is to drive 4 leds from a 7.5V source, the best way to do it in my opinion is to run two sets of 2 in series with a resistor. It uses the fewest parts and is the most efficient way of doing the job.
I think i linked to this website in a previous post of mine, this site has an excellent calculator to help those who are just starting with electronics.
http://led.linear1.org/led.wiz
The best design is always the most simple, a PWM driver would be significant overkill for driving 4 leds at 20mA average current, any potential power saving would have to be weighed against the power used by the driver etc.
But in actual fact it would be less efficient to drive the leds with PWM, leds are always more efficient at lower currents, PWM only really becomes useful when dimming leds, as you can deliver a lower average current more efficiently than constant current regulation.
The lm317 based circuit is however quite a good way of regulating current for driving leds, i use a similar circuit to regulate my led arrays, I posted some pictures
here
Whilst cutting edge leds have luminous efficacies over 100lm/W they are very expensive, your average ebay T1 led is probably around 30lm/W and is not even as efficient as the original backlight in the psone screen even after driver losses, you only end up saving power doing the "led mod" by drastically reducing the screen's brightness.
That's not to say it isn't worth it, I reckon by using 4 leds instead of the ccfl on the psone screen you would save over 500mA draw at 7.4V, and that's pretty significant.
