4. (8 points) In a certain "Mathematical Problem Solving Ability Exhibition" reader selection activity, the preliminary round had a total of 12,000 students participating, divided into junior high school, primary high grade, and primary middle grade groups. The two primary school groups together account for $\frac{15}{16}$ of the total number of participants, and those not in the primary high grade group account for $\frac{1}{2}$ of the total number of participants. Therefore, the number of participants in the primary middle grade group is $\qquad$ people.
【Solution】Solution: $1-\frac{15}{16}=\frac{1}{16}$,
$$
\begin{array}{l}
\frac{1}{2}-\frac{1}{16}=\frac{7}{16} \\
12000 \times \frac{7}{16}=5250 \text { (people); }
\end{array}
$$

Answer: The number of participants in the middle-grade group of the primary school is 5250. Therefore, the answer is: 5250.