The remainder theorem says that when a polynomial is divided by a linear factor x - c (note the minus sign), the remainder is the value of the polynomial at x = c.
When a polynomial P(x) is divided by x - c, the remainder is P(c). In other words, to find the remainder, plug in c for x.
You're dividing by x + 4 which is the same thing as x - (-4) -- the role of c is being played by -4.
