f(x) = b (x + Cos x) + 4
f’(x) = b (1 – Sin x) + 0
f’(x) = b (1 – Sin x)
\(\because\) f(x) is decreasing on R.
\(\therefore\) f’(x) < 0
b (1 – Sin x) < 0
\(\because\) Sin x ≤ 1
1 – Sin x ≥ 0
\(\because\) b (1 – Sin x) < 0 & 1 – Sin x ≥ 0
\(\therefore\) b < 0
\(\therefore\) b ∈ (-∞, 0)
Hence the required set of values is b ∈ (-∞, 0).