Hi NotUrsula,
I hope I am telling you correct, this was my experience anyway.
40,000 points would acquire you 4 nights at the Dolphin. (10,000 per night). Simply call SPG to make your reservations. My experience was they took the points then and there, and you will have confirmation of this on your online spg account. I also had to change dates and they just moved the reservation without any hassle. Some of you other options are as follows. If you are staying more than 4 nights you can do cash and points to stretch you points more. $60 per night + 4000 points. I also am doing something I have never done before. We are arriving late the first night, so instead of using 10,000 points at the Dolphin, I am using 3,000 at the Sheraton Safari and then taking a cab over to the Dolphin in the morning.
Good luck and hope I gave you accurate info, but it is my understanding from my experience.