How best to go about this conversion?

Hi y'all.

Got a question about a conversion I'm planning to make. I have a 4AA headlamp which I would like to convert to LEDs.

I suppose I could always go the easy way and just wack in 4 LEDs in to the PR bulb base and plonk in a dummy battery, but I'm not very happy with that idea. It is not a particularly elegant solution. There will always be an extra battery left from the pack, the light will dim as the batteries grow weaker and the LEDs are subjected to 4.5V which does not do much for their life span.

Can anyone suggest a simple circuit which will take in 6V, buck it down to 3.6V and approx. 120mA (driving 4 parallel LEDs at 30mA each) and keep that output constant? Or deliver 4.0V with unrestrained current, or 120mA and unrestrained voltage. I'm not overly fussy. It can be current driven or voltage driven, or both, but it must keep the output as constant as possible over the life of the batteries.

The further constraints are that I only have what is available from RS Components and **** Smith's Electronics.

Thanks in advance to all. BTW, I'm hoping to get a brightness at least equal to or better than the Princeton Tec Matrix, so if my figures aren't going to give me that, let me know.

How about using 2 batteries and slapping in a LM2621 Eval board to bop the 3V up to 4V. Should have the same effect and will probably fit in the empty space of the 2 missing batteries (of course it'll cost an extra $21 for the board...).

That's $21 to you, but probably about $50 for me (exchange rate plus shipping). Simple solution but kind of expensive. It might almost be worth just getting the PT Matrix instead.

Thanks for that idea, but I would like to keep the price down. Maybe a homemade circuit built from locally available parts?

I suppose one way of doing it would be to use a zener dioder to regulate the voltage across the LEDs. But this is almost as bad as using a simple series resistor. Quite some waste of power.

Can anyone recommend an efficient buck circuit to keep the power supply even until the batteries are really drained? (6V is too much for the LEDs and limiting the supply with a simple resistor is just too wasteful. Setting up a dummy battery is too crude.)

Even better would be a circuit that can then convert to a boost when the overall output of all the batteries fall below the requisite 3.6V

myself i would put in the dummy and get ahold of the newer 1800ma 1.2v nmhy batteries, it would still overdrive a bit.
cause i am never far from a charger
or
i would use all 4 and put two leds in series
course that would defeet your output intent