jmc

Let the number of red 4-door cars be $x$. Since $\frac{1}{3}$ of the cars are red, there are $\frac{1}{3}\cdot 30=10$ red cars, so there are $10-x$ red 2-door cars. There are $(50\%)\cdot 30=(0.5)(30)=15$ cars that are 4-door cars, so $15-x$ of the 4-door cars are not red. We then have the following Venn diagram:



Adding all four categories, we have $$(10-x)+x+(15-x)+8=30.$$Simplifying gives $33-x=30$, so $x=\boxed{3}$.