Israel's Cabinet today decided to postpone the court-ordered dismantlement of the disputed Elon Morah settlement for four to six weeks beyond Thursday's deadline to give the government more time to build a replacement outpost nearby.

The Cabinet said that 30 acres of privately owned Arab land specifically cited by the High Court of Justice in its evacutaion order last month will be vacated on schedule Thrusday. But it added that withdrawal from an additional 145 acres of private land not included in the Arab landoners' lawsuit against the government will be put off for a maximum of 45 days.

Since the Elon Moneh settlers say they are not actively using the 30-acre tract anyway, the effect of the Cabinet's decision was to give the Elon Moneh settlers another six weeks on the hilltop site just east of the West Bank Arab city of Nablus.

That could be reversed, however, if any of the owners of the 145 acres who were not complainants in the original suit applied to the High Court to be included in the evacuation order.

The court, in a major setback to Prime Minister Menachem Begin's settlement policies, ordered the government on Oct. 22 to dismantle Elon Moreh within 30 days because the land for the outpost had been illegally expropriated for political reasons and not for compelling security needs, as the government had contended.

The court invoked the Fourth Geneva Convention and the Hauge Convention prohibitions against the transfer of civilian populations from a conquering country into an occupied territory, and it said that even expropriation of land for military purposes must, by nature, be temporary. The court also said that an outpost such as Elon Moreh cannot be designed to outlive the temporary military administration in the occupied area.

Israel, noting that it was attacked by Jordan in 1967 and that Jordon previously had occupied the West Bank with only two nations recognizing its sovereignty there, had said the Hague provisions on administering occupied territory do not necessarily apply in this, case, although it adheres to the provisions of the Conventions.

In an apparent attempt to buy time in the face in increasingly vocal protests by supporters of West Bank settlements, principally the ultranationalist Gush Emunim (Faith Block) settlement movement, the Cabinet last week sought a new legal interpretation of the court's directive.

The Gush Emunim, with logistical support from the Israeli Army, erected Elon Monah in one day last June before the Arab landowners could file for a restraining order. Gush Emunim has since said it will forcibly resist any attempt by the Army to evict the settlers.

Attorney General Yitzhak Zamir advised the Cabinet that according to a strict interpretation, the Nov. 22 court deadline for evacuating Elon Moreh applies only to the 30 acres owned by 17 Arab residents of the nearby village of Rujeib, and not to the remaining 145 acres of private land, even though the court opinion repreatedly referred to the dismantlement of the outpost.

Since no time limit was mentioned for the remaining land, Zamir argued, the government could postpone eviction for a "reasonable" period. Four to six weeks, the Cabinet decided, is the time needed for the Army to prepare the new site at Jabel Kebir, about five miles away.

Tamir agreed, however, that eventually the entire site must be evacuated because the court declared Elon Moreh to be an illegal encampment.