Depending on how many points you have you might find it easier to save your cat 7 for later in the year (from memory they are valid for a year, but I'm no expert - normally you could also roll-over the certificate although because of the change in points cost this won't be possible in 2009) and then use a seperate 3/4-nighter you book with your points.
You can also redeposit the Cat 7 hotel stay for X points (not sure exactly how many - you'd probably have to call Marriott as it depends on the mileage type etc) and then buy a new 4/3pointer - that might work out better value than 'wasting' the 7 nights on a 4 nights - but that might only save you 10k or so points.