First, take a look at the vertical axis.  Its. equation is x=0.  count the number of scale divisions from the origin to the point where the line crosses the vertical axis.  It's 4.  Thus, the vertical intercept is (0,4).
Next, determine the x value where the line crosses the horizontal axis.  It's 4, and so the horiz. intercept is (4,0).
Going from (0,4) to (4,0), x increases by 4 and y decreases by 4.  Thus, the slope of the given line is m = rise / run = -4/4 = -1.
Starting with y = mx + b, substitute the knowns:  
b, the y-intercept, is 4.  m, the slope, is -1.  Thus, the equation of this line in slope-intercept form is y = -x + 4.