The equation is x=-7.
The graph of x=4 would be a vertical line which means it has a undefined slope. If the equation you are trying to find is parallel to x=4 then it is also vertical and undefined. Since it passes through the point (-7, 3) and is a parallel line, the x value will always be -7. I don't think that you could write an equation in slope-intercept form or that there is a relationship between x and y.