I am looking at booking a 9 night trip next year, but am also considering a cruise...if I book the 9 nights and then end up booking a cruise, am I able to cancel 3-4 nights of the DVC reservation or do I have to cancel the entire trip and rebook?
You can cancel days off the start and/or end of your trip. You have to call MS to do it, you cannot do it online. The usual cancellation rules apply to the returned points.
Be mindful of your point status and cancelation date for banking if required. To modify your reservation you can also just send MS instructions via the member website.