Never, you don't get to have 4 stars. Your choices are 3, 5, and 10. 4 stars means that when you give a movie more than 50% positive but less than 75%, you have to give it portions of a star. And giving half a star is just silly. Half a star causes all kinds of problems astronomically. You might as well move onto another system that allows for that kind of division. Either add more stars to your repertoire or find a new system. Slices of bread or something.
LOL, I just saw someone else using 4 stars here and I see that in the newspaper
But I guess if I gave percentages, maybe a 60% or 6/10 stars. Or I could just borrow some slices of bread from the lady in your sig or something.