I have been cruising with my kids (now 11 and 15) since they were much younger than 4. I am sure you child will be "over the moon" as Royal Caribbean has an excellent kid's club and very good counselors.
Also, I wouldn't worry about how many 4 year olds are onboard...and there will be others...because, as you know, at that age children tend to think about themselves, so with an engaged counselor and a couple of new friends, life will be good.
The one thing I did find when my kids were younger is that they tend to worry about getting back to the kid's club because they are having this or that "favorite" thing. However, at 4 they do tend to get tired so there can be "overload" between the club and time on shore coupled with late nights...for them.
So with a bit of balance it should be a great time. I wouldn't worry.