There are various ways to 'trick' a voltage regulator IC into providing a different voltage. Many of these methods involve putting a potentiometer across strategic spots.
It's only by chance that you found that a .3 V germanium diode will help you out in this case.
Something else that can be done is a silicon diode inline with the 4V supply (assuming the diode has a .65 V threshold). You won't need the regulator IC. This will give you about 3.3V at your device. If it's a little low then you'll look for a different diode which has less voltage drop.
Or if it's a little high you can add a series resistor to take up a little more V drop. Regulation won't be quite as good, however, if your load resistance varies much.
Take the above with caution if there's a chance your device could be ruined by under or over-voltage.