(b) a separate set of trustees, not exceeding 4, may be appointed for any part of the trust property held on trusts distinct from those relating to any other part or parts of the trust property, notwithstanding that no new trustees or trustee are or is to be appointed for other parts of the trust property, and any existing trustee may be appointed or remain one of such separate set of trustees, or, if only one trustee was originally appointed, then, save as hereinafter provided, one separate trustee may be so appointed; and