A study at Boston Medical Center and a Providence hospital discovered that four out of every 10 men and women who were infected with the AIDS virus did not tell their sex partners about it, according to a report to be released today.

Almost two-thirds of HIV-infected patients who withheld the information from partners also did not use condoms during some sexual encounters, the researchers report in today's issue of the Archives of Internal Medicine.

"The public health message . . . is that if you don't know the HIV status of your partner, you should be having safe sex" by using a condom, said the lead researcher, Dr. David Stein, director of HIV medical activities at Brown University Medical School in Providence, R.I.