I just read the new details of the marriott rewards program starting on 1/15/09. The amount of rewards needed is shown as follows
Under a category 6, for 4 nights that is going to be 120,000 points, and right now it is 95,000 points. For 5 nights (as you get the 5th night free) it is going to be 120,000 points, but right now it is 110,000 points for 5 nights.
Basically, start booking your travel now on a standard reward as anything booked before 1/14/09 will use the old points system, and as you can see, it is a lot less points for the same rewards.