For 4.5 Volts (3 AA alkies in series), use a 33 to 47 ohm 1/4 watt resistor, or for 4.8 to 5V (4 AA NiMH in series) use 68 to 75 ohms 1/4 watt in series with each LED. They can be direct driven from 4.5V, and will be brighter than normal, but this can shorten LED lifetime. A lot of the cheaper LED flashlites do just this. Resistors are the cheapest and simpleist, but not the most efficient way. For running off 1 or 2 AA cells, there are various step up circuits, the simplest of them being the "SATCUR" circuit discussed elsewhere on the forum. One NPN transistor, a resistor and a small toroid with two 20 turn windings. Then there is the "Brinkmann" 2 transistor circuit, also discussed elsewhere on CPF.
/ed brown in NH