In order for the equation not to change, the point (0, 0) must be on the original line and so on the line after dilation. The only equation with (0, 0) as a point on the line is y=4x.
Dilation about the origin moves all points away from the origin some multiple of their distance from the origin. If a point is on the origin, it doesn't move. We call that point the "invariant" point of the transformation. For the equation of the line not to change, the invariant point must be on the line to start with.
