Sec. 4. (a) If a fiduciary disclaims a power held in a fiduciary capacity that has not been exercised, the disclaimer takes effect as of the time the instrument creating the power becomes irrevocable.
(b) If a fiduciary disclaims a power held in a fiduciary capacity that has been exercised, the disclaimer takes effect immediately after the last exercise of the power.
(c) A disclaimer under this section is effective as to another fiduciary if the disclaimer so provides and the fiduciary disclaiming has the authority to bind the estate, trust, or other person for whom the fiduciary is acting.