Unless you are very susceptible to motion sickness and/or you have very rough weather, you probably will find that you do not get sick on a cruise ship. They are very stable, and you probably won't even notice that you're on a boat when you are below deck.
That being said, the patches are effective. They do, however, have side effects... dry mouth is most common (almost 2/3 of people), and about 17% have drowsiness (I am a pharmacist, and I am citing this from a professional website).
The patch has not been studied in children and I DEFINITELY would not use it in a child. The dose if for an adult, not a child, and there is no way to cut it down even if you wanted to. The side effects would be magnified in a child. Safety is unknown.
Personally, I would get an OTC antiemetic with meclizine in it as the active ingredient. (There are several manufacturers, under different brand names... read the label for the active ingredient). You can actually take it just once a day for motion sickness and it is quite effective. The caveat is you would have to take it before you felt sick, but since you have to put the patch on at least 4 hours ahead of time, you were already planning on doing that. Ask your pediatrician what they would recommend for a 4 year old.