You can do this online as one reservation, but before walking you through that it is really easier just to make two different reservations. The hotel will not make you change rooms. Or call Marriott and have them do it for you, although call times are taking forever.

To do this online:

• Go to Marriott.com and search for your four dates at the location making sure you have checked the "Use Rewards Points" option.
• You will be taken to the page that shows all options in the city. Choose your hotel by clicking "Continue" next to the option.
• Find the room you want and click the "+ Customize Cash + Points" on the left below the room picture.
• The menu that appears defaults to points for each night. For the nights you want to pay click the "Cash" radio button.
• When you are done click "Select" and finish the reservation as you would any other.