Regulating voltage

This may be a simple question, but I was wondering if anyone could help me figure out how to take a 5 volt input and regulate it down to about 4 volts. If at all possible it would be great if this were in some way adjustable so i could have a voltage output range of 0 to 5 volts.

Thanks

try a zener diode in series

You've not said how much current you need, what level of regulation you need and what kind of environment it's in.

Most regulators do not work well with no or little voltage across them. 5Vin 4.5V out. The are low drop out regulators but a standard regulator needs 1 to 1.5V across it.

Most adjustable regulators do not work well with output voltages below 1.5 volts. If going to 0 volts is important then that can be done it you have to have it.

Low dropout voltage and adjustable to o volts is double hard.

Please, what are you trying to do? Where is the 5V coming from?