After three months of poor sales, the retail business got worse in April, the nation's big chain stores reported yesterday.

Results from four of the biggest middle-class merchants showed sales were off 6 percent at J.C. Penney, down 8.5 percent at Montgomery Ward, up only 0.7 percent at F.W. Woolworth, and ahead of last year by a bare 0.002 percent at Sears, Roebuck & Co.

"My guess is that April will prove to be the worst month of the year," said Stuart Robbins, first vice president of Paine Webber, Mitchell Hutchins. Other retail analysts agreed, saying pre-Easter blizzards snowed in consumers whose buying habits already had been chilled by the recession.

"The very adverse weather conditions killed some retailers' Easter," said Walter Loeb, a Morgan Stanley & Co. vice president who follows retail stocks.

"The weak sales were very much as expected, with the hostile economic environment and consumers cutting back on spending," he added.

The inflation rate is now down to 5 percent, so chains have to do at least that much more business just to stay even. Not many of the big retailers were able to do so.

The most impressive gain among major merchants was the 12.9 percent increase reported by the Zayre discount chain, with sales of $155 million. Discount stores are believed to be benefitting somewhat from the tendency of consumers to "trade down" to lower-priced merchandise when times are tough.

K mart Corp., the largest discounter and the retail chain second only to Sears in total volume, posted sales of $1.32 billion, a 6.7 percent gain, the same as the previous month. Until March, however, K mart had rung up double-digit sales increases for 11 months in a row.

Sears' April sales totaled $1.446 billion, only $2 million more than in March. Penney's 6 percent decline cut its sales to $797 million. At Montgomery Ward, where sales have gone down for six of the past eight months, the 8 1/2 percent drop in April produced sales of $406.6 million.

May Department Stores of St. Louis--whose Hecht Co. is the biggest department store chain in the combined Washington-Baltimore market--had a one percent increase to $249.4 million.