The equation for the x-axis is y = 0, as all points on the x-axis have a y-value of 0. A horizontal line that runs through, say, (4,1) would have an equation of y = 1 and be parallel to the x-axis.
A line perpendicular to the x-axis would have the opposite. The y-axis, for example, has an equation of x = 0 because any point on the y-axis has a y-value of 0.
Therefore, the equation of your vertical line passing through (4,1) is x = 4. Any other point on that line has an x-value of 4.