Italy's 40th government since World War II has taken office. Arnaldo Forlani , a former foreign minister and now president of the dominant Christian Democratic Party, succeeded in putting together a four-party coalition including the Socialists, Social Democrats, and Republicans. All but the Social Democrats figured in the last government. The three-week gap between the fall of the last Cabinet and the announcement of the new government was brief by Italian standards, reflecting a concern among all parties about the effect of an extended crisis on the economy and the lira. A key factor in Mr. Forlani's success was a parliamentary pact between the Socialists and Social Democrats which reflects the emergence of a strong center-left group willing to cooperate with the Christian Democrats and to exclude the Communists.