4. Given the inequality $a x+3 \geqslant 0$ has positive integer solutions of $1,2,3$. Then the range of values for $a$ is $\qquad$
4. Obviously $a<0$, so $x \leqslant-\frac{3}{a}$. According to the value of $x$, we get 3 $\leqslant-\frac{3}{a}<4$. Solving this, we get $-1 \leqslant a<-\frac{3}{4}$.