Yes, you may want to call them - I'm a bit skeptical that 3 points nights and one paid night will result in a 5th night free. But the whole Cash and Points thing is a bit of an abomination anyway, so who knows.

I believe you can make reservations with points you don't have, but make sure you're clear on whether it will be one or two nights paid with cash if you don't get enough for the 4th night. Also ask what happens if you don't have enough, they cancel your 5 night stay, and don't have reward availability for even the three nights at that point. Call and ask the relevant questions, and get it in writing if possible.