The rule is not hard and fast, as there are different levels of "using external assets."
If all of your art is from free art packs, you'll definitely cap at 4/5 stars.
If you use a single texture but everything else is newly created, we might not subtract a full star, at the discretion of the person judging your game.
So you might get something like 4.9 for art if your art is mind-blowing but you used a single downloaded texture. :)