And maybe this isn't a new idea. But we certainly had fun doing it.
All you need: craft paint, painter's tape, canvas of any size, scissors, sponge brush(es) or cotton balls.
Decide whether you want to tape off a number (4 for the 4 year old!), a letter or several (initials) or a whole name. This is the painstaking part of the project. Take your time to get this looking right. Curves are difficult but do-able.
Make sure to really go along the edges with your fingertip repeatedly. You want a really good seal so that paint won't go under.
Then have fun with the paint. You can use sponge brushes or cotton balls or something similar. The key is not to get too much paint on your instrument, as you don't want leakage. Don't use water with the paint either.
In this case, we found out what the birthday boy's favourite colours were, and that gave us our palette. My daughter painted one, and I the other. They are a bit different, but still work together. We just smooshed with cotton balls, but you could do stripes or any kind of pattern as well.
Tada!