電源供應器電壓問題?

直接定做一個直流4V的電源供應器.

You have to add a control circuit to it. For the simplicity, you can just use a voltage regulator L7805, so that the output voltage is regulated at DC5V. If you want the exact 4V, you have to add a resistor in between to form a voltage divider. But in this case the working current has significant impact on the stability of voltage regulation.