Wow! Four 7-nights? Or shorter? ... to answer your question, as long as you book the same stateroom for each cruise, you will not need to pack up. You will need to leave the ship and officially check-in again for the next cruise, but sans luggage. If you book different staterooms, you will need to pack but your stateroom host will handle moving the luggage to the next stateroom for you.
Enjoy your cruises!