What is the ruling on a grandmother writing a agricultural land to her four daughters—one of whom is the mother—in order to deprive them of sharing in the inheritance with her male nephews after her death, while she continues to benefit from the land’s yield until her death?

It is not permissible to make a gift (hibah) or a bequest (wasiyyah) with the intention of harming the heirs and depriving them. What is mentioned in the question is a bequest, not a gift, and what matters are the meanings, not the words. A bequest to an heir is not valid unless approved by the other heirs. The grandmother must fear Allah, for harming the heirs is forbidden by Islamic law.