Plan on packing for an eight day cruise and sending out your laundry one time.
You can always re-wear many things that don't become dirty or soiled by perspiration. I wouldn't over wear them but a couple times is fine.
Under clothes don't take up much space and can also be rinsed out.
Biting the bullet and sending out the clothes for laundering may sound expensive but it's a much better alternative than lugging and managing four suitcases.
One other thing: People really won't notice if you wear the same formal outfit one of the three formal nights.