To find the y intercept you need to write the equation in its usual form y=mx+c where m and c are constants (numbers). So -y=x+4 is put together not the right form by multiplying both sides by -1:
y=-x-4. The constant -4 is the y intercept.
Another way is to put x=0 into the equation so that we get -y=4 and, again, we swap the signs by multiplying both sides by -1: y=-4, the y intercept. It’s called the y intercept because when we plot the graph of the equation we get a straight line which crosses the y-axis at y=-4.