the line y = 2x -3 has a gradient of 2.
so the line perpendicular to it has a gradient of -1/2 and will have the general formula of y = -1/2 x + c.
to find c, use the coordinates (0,4)
4 = 0 + c
so c = 4
equation is y = -1/2 x + 4
If you need the distance of (0,4) from the line you will need to put both lines on a graph to find the intersection (and possibly use simultaneous equations for more accurate answer) and then use pythagoras to find lengths.