4. (5 points) Hongfu Supermarket purchased a batch of salt. In the first month, 40% of the salt was sold, and in the second month, another 420 bags were sold. At this point, the ratio of the salt sold to the remaining salt is 3:1. How many bags of salt did Hongfu Supermarket purchase?
【Solution】Solve: $420 \div\left(1-40 \%-\frac{1}{3+1}\right)$
$$
\begin{array}{l}
=420 \div 0.35 \\
=1200 \text { (bags) }
\end{array}
$$

Answer: Hongfu Supermarket purchased 1200 bags of salt in this batch. Therefore, the answer is: 1200.