President Reagan cited four "very substantial changes" in South African society that, he said in a radio interview released by the White House yesterday, had "resulted . . . from our present relationship" with the South African government. Glenn Frankel of The Washington Post Foreign Service addressed each of those points in a dispatch from Johannesburg yesterday:

Reagan said: ". . . Now the blacks have ability -- being in labor unions or even having their own labor unions."

Black South Africans can belong to labor unions. However, that achievement cannot be credited to the Reagan administration's policy of "constructive engagement." It was legislation passed in 1979, when Jimmy Carter was president, that gave blacks the right to join unions, though the government sharply restricts the rights to bargain collectively or go on strike.

Reagan cited "the fact they [blacks] can buy property in the heretofore white areas, that they can own businesses in some 40 white-dominated business districts."

The president may have been referring to a government policy, implemented about four years ago, that allows some blacks to own houses and acquire 99-year leasehold rights for the land under them in black townships, such as Soweto, that are located in the 87 percent of South Africa that is considered white territory. So far, that policy has been mired in red tape, and only in Soweto have blacks been able to acquire leaseholds in significant numbers. The government is considering converting those lease rights into the right to actually own the land involved.

As for businesses, a government policy announced last year but yet to be enacted into law would allow blacks to open businesses in white central cities where most urban blacks shop. But the properties would still be owned by whites.

Generally, South Africa's Group Areas Act, one of the pillars of the apartheid system of racial segregation, clearly defines by race the various residential and business districts in the country and restricts each race group -- including Indians and so-called "coloreds" -- to their own designated areas. They cannot own or rent property outside their designated "group area." The Influx Control Act, another pillar, requires blacks to have written government permission to enter "white areas."

Two weeks ago, Minister of Constitutional Development Chris Heunis reassured whites at a meeting of members of the ruling National Party that the Group Areas Act would not be scrapped.

Reagan said: "They [the South African government] have eliminated the segregation that we once had in our own country -- the type of thing where hotels and restaurants and places of entertainment and so forth were segregated -- that has all been eliminated."

In fact, hotels and restaurants must have special government permits to serve blacks. Perhaps two dozen hotels and three dozen restaurants in Johannesburg have such permits, while many hotels and restaurants remain off-limits to blacks. Most sports grounds and some stage theaters are integrated, but there are no integrated movie houses in South Africa. This leads to the incongruity of places like the Sandton City mall in suburban Johannesburg where blacks can stay at a luxury hotel but cannot attend the movie theater next door. This permit system began under the government of the late prime minister John Vorster a decade ago, long before Reagan took office. "Jim Crow" restrictions imposing strict segregation in buses and trains still apply, although airplanes have been integrated for several years.

Reagan said: "They [the government] recognize now interracial marriages and all."

The law banning mixed marriages was repealed this year, and many commentators here credit quiet lobbying from the Reagan administration plus the desire of the South African government to bolster constructive engagement as important factors behind the repeal. However, a black married to a white cannot legally live with his or her spouse in a white area.