Y-intercept 4 mean that your line pases through (0;4)
Your formula is:
\(y=mx+b\)
So we have (x;y) which is (0;4) and slope which is m=-2
Let s put it into our formula:
4=-2*0+b
b=4
Your equation:
\(y=-2x+4\)
Here s your graph:
https://www.desmos.com/calculator/cjyxjbsaft