San Antonio Mayor Henry G. Cisneros, a leading Latino politician, said he has been in public service long enough and will not seek a fifth term. His term expires next spring. Cisneros was first elected mayor in 1981. Democratic presidential nominee Walter F. Mondale interviewed Cisneros in 1984 about possibly running on his ticket. Last year, Cisneros, 41, said he would not seek a statewide office because he wanted to devote time to San Antonio projects and to his son, John Paul Anthony, who was born with heart and stomach abnormalities. Cisneros was approached this year by several San Antonio businessmen about forming a business investment firm. The mayor’s salary is less than $5,000. Cisneros said he needs a different job because his two daughters are approaching college age and because of his son’s medical expenses.