I am making a robot with 4 transistors and 3 resistors. But for the circuit to work, I need 4 volts and 2 watts. How would I connect batteries to get that?

 

6V battery with a LM317 regulator adjusted to 4.0V is about the easiest way I can think of. It is quite possible your circuit will work directly off 3AA cells in series (4.5V)

 

Most things that say 4 volts don't mean 4.00 volts or I'll blow up. Then there are a lot of people that have an LED labeled 4 volts maximum and they blow it up because they won't tell us what they are doing and they don't know that LEDs care about current, not voltage.

 

Three NI-CAD or NI-MH will be 4.2V - 3.7V over their full life.

 

How long do you need this 2W to last?