Suppose y = y(x) be the solution curve to the differential equation \(\frac{dy}{dx} \) - y = 2 - e-x such that \(\lim\limits_{x\to \infty} y(x)\) is finite. If a and b are respectively the x- and y- intercepts of the tangent to the curve at x = 0, then the value of a - 4b is equal to ______.