Easiest would probably be buy SPG points since its 35% off right now so 30k for $682.50. 30k*3 = 90k Marriott points. So 341.25 per night for Category 9.
But thats probably not worth it for you since that hotel is a Category 9 and looking at the price, its 220 per night.
The Category 9 I usually book is 400-500 a night so its worth it for me to buy points but I'm not sure in your case.
If you're only short a little bit to get the 4th night, you can buy the points to get 4 nights and get the 5th night free and pay for the 6th night with cash. The price is 30-40 dollars more than normal rate doing mix of cash and point but that option might be better if you're short a few points.