I’ve been playing New Super Mario Bros. U last night and today, and I’ve now collected all of the star coins in the game.
As a result, I got a fourth star on my file.
To get the fifth and final star, apparently you have to find all 94 exits. So that means there are still two secret exits I need to find. I’ll probably have to consult a guide to see which two levels still have secret exits remaining, even though I want to find the actual exits myself.