The third and fourth women astronauts to be chosen for space shuttle flights were named yesterday to crew assignments, leaving only four of the eight women in the astronaut corps not assigned to a flight.

Named a member of the crew of the 13th shuttle flight, scheduled for next June, was Dr. Rhea Seddon, 35. Dr. Anna Fisher, 34, was named to the 15th flight, next August. Both women are physicians married to astronauts.

Seddon was scheduled to be assigned to an earlier flight but was taken out of the rotation when she had a baby in July, 1982, and missed several months of training. Fisher had a baby two months ago but left the hospital the day after she gave birth and returned to training five days later. Six rookie astronauts (including Seddon and Fisher) were named at Houston's Johnson Space Center to fill out the five-person crews for the 13th and 15th flights.

Picked commander of the 13th flight was Karol Bobko, 45, who piloted the sixth shuttle flight last April. Chosen commander of the 15th flight was Frederick H. Hauck, 42, who piloted the seventh flight on which Sally K. Ride became the first American woman to fly in space.

Besides Seddon, the other rookies chosen for the 13th flight are Donald E. Williams, 41, who will be the pilot; Jeffrey Hoffman, 38, a physicist who will become the first Harvard graduate to fly in space, and S. David Griggs, 43, a Naval Academy graduate who received his master's degree at George Washington University and who has served as a NASA test pilot for the last seven years.

One other rookie beside Fisher will be on the 15th flight. He is David Walker, 39, a Navy commander, who will be the pilot. Returning to space for the second time will be mission specialists Joseph P. Allen, 46, who was on the fifth flight last November, and Dale A. Gardner, 34, who made the last flight earlier this month on which Guion S. Bluford became the first black American to orbit the Earth.

Of the four black astronauts, only Charles Bolden is unassigned. Still unassigned among the eight women astronauts are Kathleen Sullivan, Shannon Lucid, Bonnie Dunbar and Mary Cleave. Beside Ride, Judith Resnick is already assigned to the 11th shuttle flight, now scheduled for next May.