Four years after the Food and Drug Administration began urging food manufacturers to use less salt voluntarily, the amount of salt in most processed foods has stayed the same, a consumer advocacy group concludes.

"Most products are as salty as they were two years ago," said Bonnie Liebman of the Center for Science in the Public Interest, "and of the ones that have changed, almost as many have gone up in sodium as have gone down."

More than 60 million Americans have high blood pressure, and many of them are sensitive to sodium and are on low-salt diets.

Of the 2,179 products measured by CSPI, 83 percent showed no change in sodium content. Nine percent went down, and 8 percent went up.

Ragu spaghetti sauces, which had a 20 percent drop in salt content in the past year, were singled out for praise by the organization. Also cited were Nabisco, whose salt levels in cereal dropped 47 percent, and McDonald's, which had an overall decline in salt of 14 percent.

Stouffer's, CSPI said, cut sodium in its pizza and side dishes by 27 percent, but increased the amount in frozen soups by 40 percent. Other increases were in Chef Boy-Ar-Dee Lasagna (up 30 percent) and Quaker Oats hot cereals (up 24 percent).

A spokesman for the Food and Drug Administration disputed the CSPI conclusions. Since 1978, he said, the number of foods with salt labeling increased from 7 percent to 48 percent, and new low-salt foods are being introduced at a rate of four a month, compared with four a year in 1981.

"I think the consumer's covered from every possible angle," the spokesman said.