Fill in the squares with the numbers $1,2,3,5,6$.

$$
(\square+\square-\square) \times \square \div \square=4
$$
The operation is equivalent to

$$
(\square+\square-\square) \times \square=4 \times
$$

Therefore, the left side of the equality is a multiple of 4, so the only possibilities are:

$$
(\square+\square-\square) \times \square=4 \times \square \quad \text { or } \quad(\square+\square-\square) \times \square=4 \times \square
$$

From this, we can conclude that:

$$
(\boxed{3}+5-\square) \times \square=4 \times \square \quad \text { or } \quad(6+5-\square) \times \square=4 \times \square
$$