An agent, unless specially forbidden by the principal to do so, can delegate the agent's powers to another person in any of the following cases and in no others:
(1) when the act to be done is purely mechanical;
(2) when the agent cannot and the subagent can lawfully perform the act;
(3) when it is the usage of the place to delegate the powers; or
(4) when the delegation is specially authorized by the principal.