Guidance in micro project for driving LEDs

Other Hardware LEDs and Multiplexing
1

Hello,
I have a simple circuit that drives some LEDs; basically the arduino switches a MOSFET that has the LEDs on the drain, with the source connected to ground.
In series with the LEDs I have a resistor for limiting the current.
The LEDs have a Vf of 3.2V and I'm concerned about battery life (they have a battery pack supplying voltage to them, independently from the arduino).
I'm asking, then, which is best to ensure the longest life of the batteries (and minimal waste of power)?
I mean: is it best to have the power source at 4V and a small resistor or to have a 9V supply and a larger resistor, or will it be the same?

Thanks, I'm kind of a noob on these topics... :-[

2

All LEDs turn on together?
Wire a couple in series with a current limit resistor for 5-10mA or whatever you set the current to. The LEDs than use the same 5-10mA current to turn on.
Make up as many strings as you can, all connected in parallel, powered from 9V (6 x AA battery pack for example).
With individual LEDs, need a current limit resistor for each, and each will use 5-10mA, or whatever you set the current to.

3

To maximize efficiency, minimize the amount of voltage that the resistor has to drop. The resistor is wasting that power (as heat).