A spokesman for Rupert Murdoch said his Fox Television Stations registration statement, offering to exchange cash or preferred stock for about $1.9 billion in debt securities of Metromedia Broadcasting, has been declared effective by the SEC . . . Federated Department Stores said its Chicago-based MainStreet division will open four new stores in the Chicago and Detroit markets this fall and in 1987 . . . Standard & Poor's said it lowered Revlon's senior debt rating to B from B+ . . . CalFed Inc. reported that it will increase its issue of convertible subordinated debentures in the Euromarket to $125 million from $100 million . . . American Motors said it is extending its low-interest financing and cash rebate promotion of Renault Alliance and Encore models through next month . . . The Investment Company Institute reported that the assets of the nation's 363 money-market mutual funds fell $742.5 million in the week ended Jan. 29.