""Question. If I only want to do the constant current charge till 4.0v what must I limit my voltage to? I'm slightly confused because a constant current adjusts the voltage to keep a constant current. Will this not cause the charging voltage to go above 4.2volts?""
You have already answered your own question. You raise voltage to keep a constant current but once you reach 4.2 you revert to constant voltage - hence voltage never exceeds 4.2v