Prevent voltage "backflow" from USB 5V thru VIN

I'm using an Arduino Pro Micro. When it's powered only via the USB, the RAW pin has ~4.5V on it. I have it designed so that it has the potential for 9V into the RAW, but in the scenario where the user only supplies the voltage via USB, I do not want any of my other devices to get any power.

Can I use a diode or anything to prevent 4.5V from coming "out" the RAW/VIN pin?

Yes, a diode will do that.

Any particular, recommended diode? Would a 1N5817 work? Will it burn up if voltage is passed the other way?

If you have one, use a schottky diode. They have a small forward voltage drop

hansibull:
If you have one, use a schottky diode. They have a small forward voltage drop

The 1N5817 is a Schottky, right?

For what it's worth, I tried the 1N5817 and it "worked," but I learned that the reverse leakage was a bit higher than I would have liked.

I switched to a 1N4004 and it seems to work great.