Depends on how much you want to spend. We have travelled on a variety. Ocean village has great size rooms and excellant children facilities. Our friends had their 2 year old grandson with them! The Family rooms with bunks are a bit tight so perhaps a normal double with an easy bed would be better. Much nicer rooms.The entertainment is a question of taste. Look at the number of places cruises stop at. In the med. you can do a lot by yourselves by just doing the leg work before you go. For example the port for Rome you can get a train from which costs about 9 euros whereas the trips on board are from £50 per person!!!! Even going to Pisa is easy by train but if you want to go up the leaning tower pre book your tickets on line.
We have also seen the children on Island Cruises thoroughly enjoying themselves.
Remember if you are out of school holidays there may not be as many children as you might like for your 4 year old to play with but the staff are full of enthusiasm.
Easy. RCI Brilliance of the Seas, beautiful ship and wonderful itinerary out of BCN. Or you could try the new Celebrity Solstice which looks totally beautiful - http://www.depping-design.de/test/thumbnails.php?album=92.
The Grand princess sail out of Southampton next year.If it is like its sister ship SEa PRINCESS you and your family will love it.Lovely sized cabins great food great entertainment and above all the children are well looked after if you decide to put to let your child go to the morning activities.Room service if you need it anytime night or day.Just one thing i would reccomend a cabin with a window.