If the 4 leds are in series, and if they use a plain resistive drive
circuit, and if you know the current requirement and the voltage for
each led, then the resistance required to be in series with the string
of leds would be: (12volts - 4*voltage_per_led) / amps.) (600 ohms at 2
volts and 100 ma), I seem to recall. A variable 1000 ohms with a 60 ohm
limiter in series would give a dimming effect. The light will be
reponsive to voltage fluctuations. You could start with a 1000 ohm
rheostat and measure the values at the performance you want. In
reality, the light will vary with the supply voltage.

The series curent limiting resistor is calculated on the excess drive
voltage.

Most new led assembly tricks use a small chopper chip to drive the
leds at max rated current for short duty cycles, to save power and
protect the leds.

Some leds are diffused, some are spots, requiring a cone shaped
reflector to diffuse the light be made up from tinfoil, etc.

Terry K