An enzyme follows Michaelis-Menten kinetics with $V_{\max} = 200\ \mu\text{mol min}^{-1}$ and $K_m = 2.0\ \text{mM}$. At a substrate concentration of $4.0\ \text{mM}$ it works at a certain rate. A competitive inhibitor with $K_i = 1.5\ \text{mM}$ is then added at $3.0\ \text{mM}$. To bring the rate back to its original value, the substrate concentration must be raised to what value, in $\text{mM}$? Give a whole number.