4. Find the isolated singularities of the following functions, and determine whether they are removable, essential, or poles. Also find the order of any poles.
(a) f(z) = cos(z) / (z^2 - R^2)
(b) f(z) = (z - 1) / (z^2 + 1)
(c) f(z) = z^3 - 1 / (z - 1)
(d) f(z) = z^2 + Mp sin(z)