Ac=4 bx=y+4 doesn't make sense. Perhaps you meant Ac-4bx=y+4. If this is not correct, ensure that you have copied down the original problem correctly.
To solve Ac-4bx=y+4 for y, subtract 4 from both sides of this equation. You'll get:
Ac-4bx-4 = y+4 - 4, or y = ac-4bx - 4.