You shouldn't be driving LEDs from fixed *voltage* sources. They need to be either current sources (best) as the makers spec them, or at least with enough series resistance to give some stability.
Not only does Vf at a given current vary from device to device but it goes *down* (the wrong way) with both heat and time. Not to mention what can happen when the supply is even a little bit high. The net effect of which is to either degrade your light or outright kill it far too early in life.
There's a reason why the makers don't recommend (or even support) this drive, and why all the well engineered lights you see aoid it as well. Even those that seem like constant voltage (like say key chain type 'coin cell' lights) are counting on system resistance you're probably not even considering.
So, yes, they can be run from four volts, put a resistor in series. Better if it's a little higher so there'll be more voltage 'across the resistor'.
I have LEDs that run about 40-50ma at 4v but Doug is right, they vary a lot in Vf. If you can come across some with higher forward voltage they would probably run at that range. Are you sure you are not using a boost circuit or multiple LEDs when you measure that 150ma current draw? I figure that high of current would fry a standard 5mm LED. I do not recall seeing anyone able to run one much above 90ma for any period of time. And the ones running that high were I believe nichias.
[ QUOTE ] davidefromitaly said:
so i must use a 5ohm resistor?
only i have a very little-little space for fit the resistance...
[/ QUOTE ]
I'd try more like 15 or 20. It doesn't have to be big physically. You really don't want more than about 30 mA, in fact past 60 or so total light goes *down* with more current, as does efficiency and battery life. Device life too for that matter, and light output falls off before it quits all together. The harder you push them (heat), the faster it happens.