To find the equation of the line in the form \(y = mx\) that passes through the origin and describes the proportional relationship based on the given points \((1, 4)\) and \((2, 8)\), we first need to determine the slope \(m\).
The slope \(m\) is calculated using the formula:
\[ m = \frac{y_2 - y_1}{x_2 - x_1} \]
Using the points \((x_1, y_1) = (1, 4)\) and \((x_2, y_2) = (2, 8)\):
\[ m = \frac{8 - 4}{2 - 1} = \frac{4}{1} = 4 \]
Now that we have the slope, we can form the equation of the line through the origin:
\[ y = mx \quad \Rightarrow \quad y = 4x \]
Thus, the equation of the line is:
\[ \boxed{4x} \]