Four billion dollars a year. That's how much it would take to make dramatic gains in slowing down population growth in the third world, according to experts who attended the just-ended world population conference in Mexico City.
''Compared to world spending on arms (more than $900 billion a year), it's minuscule,'' says Dr. Joseph Speidel, vice-president of the Population Crisis Committee in Washington.
The money would be enough to provide basic family-planning services to every one of the estimated 400 million couples in the third world (excluding China) aged between 15 and 49. It is about four times the total being spent today outside China.
''Family-planning spending is also extremely cost effective,'' said Dr. Speidel in an interview as the population conference ended on an optimistic note. It costs about $10 a year to provide a third-world couple with basic family-planning services.
''We estimate that, over the past 15 years, existing family-planning programs , as limited as they still are, have helped to avert an estimated 130 million unwanted births. That's a savings in food, shelter, clothing, health care, and education costs of some $175 billion.''
There is little immediate prospect of a four-fold jump in family-planning spending, given existing third-world debts, the state of world trade, and high interest rates and unemployment in industrial donor countries.
But a number of third-world and other delegates to the Mexico City conference took encouragement from:
* A pledge here by World Bank president A. W. Clausen ''to at least double our population and related health lending over the next few years.''
In the last 14 years, Mr. Clausen said, the World Bank had committed about $ 500 million for population projects. New efforts would concentrate on Asia and Africa, where the bank aims to finance more than 20 projects south of the Sahara in 17 borrowing countries.
* An affirmation by the leader of the United States delegation, James Buckley , that the Reagan administration intends to spend the full amount of money voted for family-planning aid by Congress each year.
The US figure is $240 million for the fiscal year ending Sept. 30. An appropriations subcommittee of the House of Representatives has just voted to raise this to $290 million for fiscal 1985, though the figure could be reduced in conference with the Senate.
* The firm commitment to increased family planning in the final documents emerging from the Mexico City conference, including a Mexico City Declaration and a list of recommendations approved by general consensus.
''I am pleased with the final recommendations,'' said a Kuwaiti delegate minutes after the conference had ended dramatically with the US delegation agreeing to all population decisions (but rejecting recommendations on disarmament and the Middle East).
''This is the US we used to know. For most of this conference the US stood alone. Now, on the last day, you became responsible again. You let the final documents pass, and now we have a consensus that family planning is indeed a part of economic development.''
It will not be easy to find extra donor funds for family planning, however.
''US aid programs in this field are underfunded right now by 50 percent,'' says Rep. Sander Levin (D) of Michigan, who specializes in the issue.
Mr. Levin, formerly with the US Agency for International Development (AID) and in Mexico City to attend a world parliamentarians meeting on population Aug. 15 and 16, added: ''We need a substantial increase in donor funds. But there's no magical target figure to aim at. We have to see what developing countries actually want, program by program.''
Seven years ago, Mr. Levin said, rich nations had enough money for all family-planning projects requested by the third world. Now requests greatly exceed aid funds available.
About $1.1 billion is spent on family planning (outside of China). One-quarter comes from rich nations, the rest from the countries involved. Including China, world spending is about $2 billion, of which rich nations provide $490 million. The US is the largest government donor.
The head of one of the most successful programs is Haryono Suyono, chairman of the National Family Planning Board of Indonesia. While attending the conference here, he said that a one-half increase in spending would have dramatic results.
Almost 60 percent of couples aged 15 to 49 use contraception in Indonesia. Mr. Haryono thinks that an extra $50 million a year in spending (on top of an annual budget upwards of $100 million) could lift that to 70 percent in three to five years.
''We need to decentralize even more into our villages,'' he said. ''Our eventual aim would be to have the government hand over family planning work to the people themselves, to private groups ... but government help is still needed for the moment.''