Just thought I'd chime in about where that LED equation comes from if you are not familiar with electronics.

If the forward voltage of the LED is nomially 3.0V, the other 9.0V of your 12.0V supply needs to be dropped across the resistor. If you want 20mA of current flowing through the LED (as well as the resistor, considering they are in series) you would use the equation from Ohm's law:

Rled = Vres / Iled where Vres = Vsupply - Vled

Example:

Vsupply = 12V

Vled = 3.0V * #LED

#LED = 3

Iled = 20mA

Rled = (12.0V - 3*3.0V) / 20mA = 150 ohms

Technically, LEDs have forward resistance because they have a voltage drop, but you don't want them dissipating more power than they have to/are capable of. Also, this forward resistance is not fixed as is the resistance of the current limiting resistor you use.

in the case of an LED with a forward voltage of 3V with a recommended current of 20mA:

R = E/I = 3V / 20mA = 150ohm

This is not fixed, it varies with some function. The forward voltage given by the manufacturer is recommended for reliability and performance.

In my opinion, if you have four 3V LEDs in series across a 12V supply, you only need a minimal resistor. This ruins that equation about LED resistors because the impedance of an LED varies over the V-I (voltage - current) curve.

http://www.cq.cx/pics/int-led-vi.png

This shows the impedance at 1.8V forward voltage to be 360 ohms and 100 ohms at 2.0V. This is a wide variation, and although LEDs have resistance, it is not fixed like a resistor. Since none of the electrical characteristics for an LED are fixed (voltage, current, resistance), simple Ohm's law equations like the one you showed break down.

I would put the 4 LEDs in series and test the current with a DMM and see what resistance you need.

You will not blow up the LEDs this way, though the live may be diminished if the current is too high. I don't see a problem arising, though.

EDIT:

To sum it all up: If you put the LEDs in series and the voltage across each LED is 3V, and the power supply is fixed and regulated, you won't have a problem. fixed voltage == fixed current.