Let:
(x1,y1) = (0,4)
(x2,y2) = (4,0)
The slope m is:
[tex]\begin{gathered} m=\frac{y2-y1}{x2-x1}=\frac{0-4}{4-0}=\frac{-4}{4}=-1 \\ \end{gathered}[/tex]Using the point slope equation:
[tex]\begin{gathered} y-y1=m(x-x1) \\ y-4=-1(x-0) \\ y=-x+4 \\ \text{ In standard form:} \\ x+y=4 \end{gathered}[/tex]