hey there...
im just a begginner and im struggling on how to construct a power supply with an output of 4v.can someone help me

 

It is too simple for me to talk about it.
Maybe somebody here will teach you about the basics of electronics or design the simple power supply for you. But since you did not mention how much current it must produce then a design is not practical.

 

You need to provide further specification. The voltage and current at minimum.

 

More to the point - what is it that you need to power with this supply, and what is it's current requirement or wattage rating or VA rating?