Provided always, that the sum or sums of money laid out upon such improvements, by any one heir of entail during his or her possession, shall not, in any case whatever, be effectual to constitute a claim against the succeeding heir of entail, for more than four years free rent of the said entailed estate, after deduction of all publick burdens, life-rents, and interests of debts, which may affect the said estate, as the same shall happen to be at the first term of Whitsunday after the death of the heir who expended the money claimed.