(4) Where, in any will or other testamentary disposition, a stipulation is made to the effect that the beneficiaries therein, or one or more of them, shall not receive any income accrued under such will or disposition until the happening of an event, whether fixed or contingent, any such income as would, but for the stipulation, have accrued to the beneficiaries shall, until the happening of that event accrues to the trust and shall be included in the assessable income of the trust and the chargeable income ascertained therefrom is to be charged to tax in the name of the trustee.