What is the slope of the line x = 4?
#m = (Delta y)/(Delta x) = (y_2-y_1)/(x_2-x_1)#
In the case of the line
#m = (1 - 0)/(4-4) = 1/0#
... which is undefined.
You might try to say that its slope is 'infinite', but actually it's not clear whether that should be