How to reduce 5 volts to 4 volts 1.5Amp with minimum components used?
Consider connecting a diode in series with the load.

edit: Depending on the application, this might be enough, but the forward voltage drop of a diode (or two) does vary with the load current and the temperature of the diode(s). Also, some power (1 W per A) is wasted. Depending on the duty cycle, heat sinking for the diode(s) may need to be considered.

If you, for example, want to drop the voltage supplied to a DC motor to make it run a little slower, a diode in series is good.

edit by  Russell McMahon:  A 2A rated silicon diode or 2 x 1A diodes in series will typically have 0.6 to 1.0 Volts drop at near rated current. Actual value varies with type - see data sheet. Voltage drop increases with current. Whether this is precise enough depends on application (the O.P. needs to give more information about the application).