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).

An adjustable (because 4.0V is not a commonly used voltage) LDO regulator is the easiest route to take. For example, the Micrel 29150/29300 regulators can handle 1.5A or 3A with a dropout of well under 1V guaranteed.

You need to read the datasheet, calculate two resistors to get the required voltage and to follow the datasheet recommendations for the capacitors (especially the output capacitor) to the letter. For example in the above-linked datasheet where it says low ESR capacitors 'may contribute to instability' that means that under no circumstances should you use a large ceramic capacitor as the output cap without a series resistor. There are other options for the chip, this is just one possibility. Whatever chip will be dissipating about 1.5W at 1.5A so a small heat sink is called for. If you're actually using it at 1.5A, the 3A-rated part would be a good idea.

There are definitely 3 terminal regulators with low dropout that suit this requirement.
See eg www.digikey.com and enter search terms.
As an example only The Seiko S816 works with an external transistor and not including decoupling capacitors requires TWO components total. Dropout is limited by the external transistor's characteristics. While a bipolar would usually be used, this would drive a suitable MOSFET and dropout voltages as sensibly low as desired would be possible.

Using Digikey's search also found

MIC29302 in stock $2.86/1
3A, 250 mV dropout typical at 1.5A. Agh/whoops - I now see I've arrived at the same device as Spehro :-). Search Digikey using their excellent parameter driven search. This and more are there.

Also

Semtech SC1592 in stock $1.82/1.
260 mV dropout at 3A BUT uses a special dual input supply mode - power conversion is low dropout but Vspply_control needs to be Vout + 1.5V. May or may not be useful.