<<Here is a simple circuit using Linear Technologies SwitcherCAD program.
3.2 to 6v input, 4.0v output at up to 350mA.>>

There's something missing with the above circuit. You have a straight line from input to output through the diode. If you go over 4.3 volts input, the voltage will be over what you need.

Why must you run the PIC circuit at 3.3 volts? It won't work at 3.6?