(1) Save as otherwise expressly provided in this Act,—
(a) any text, rule or interpretation of Hindu law or any custom or usage as part of that law in force immediately before the commencement of this Act shall cease to have effect with respect to any matter for which provision is made in this Act;
(b) any other law in force immediately before the commencement of this Act shall cease to apply to Hindus in so far as it is inconsistent with any of the provisions contained in this Act.
(2) For the removal of doubts it is hereby declared that nothing contained in this Act shall be deemed to affect the provisions of any law for the time being in force providing for the prevention of fragmentation of agricultural holdings or for the fixation of ceilings or for the devolution of tenancy rights in respect of such holdings.
Legal Commentary & Landmark Judgments: Section 4
Section 4 establishes the absolute overriding and preemptive effect of the Hindu Succession Act, 1956, abrogating all inconsistent pre-existing texts, shastric doctrines, customs, and prior statutes.
Whenever an opposing coparcener sets up an ancient family custom (e.g., that married daughters forfeit ancestral shares or that only male agnates inherit), cite Section 4. Section 4 invalidates all customary exclusions not expressly saved by the Act.