Chapter P - Section P.1 - Algebraic Expressions, Mathematical Models, and Real Numbers - Exercise Set - Page 18: 121

$x-(x+4)$ $-4$

Since the number is being decreased by the sum of it and four, it would be $x-(x+4)$. The sum is the answer you get when you add numbers, and decreased implies subtraction. This then simplifies into $x-x-4$, which is just $-4$.