To find the missing number, we can set up the equation as follows:
Let \( x \) be the missing number. The equation is:
\[ x \times 1,000 = 4,200 \]
To solve for \( x \), we can divide both sides of the equation by 1,000:
\[ x = \frac{4,200}{1,000} \]
Now, performing the division:
\[ x = 4.2 \]
So, the missing number is \( 4.2 \).