On April 14, 2010, these four women were all on the International Space Station at the same time. This represents the largest simultaneous concentration of ladies in space we have ever seen.
The four women are NASA astronauts Dorothy Metcalf-Lindenburger, Stephanie Wilson, and Tracy Caldwell Dyson, and Japanese astronaut Naoko Yamazaki. While awesome to see almost a handful of women in space simultaneously, it also breaks my heart: I would love to live in a present where counting the number of women in space is a puzzling historical artifact.
The STS-131 crew are in light blue shirts, while Expedition 23 are in dark blue. Even with four women on-board at the same time, the full group and each individual crew was still less than 50% women.
NASA briefly considered sending an all-female crew into orbit in 1999. The purely-theoretical mission would have focused on women's reproductive health in space, allowing for a large enough population to conduct meaningful tests. The idea never materialized after it met push-back with accusations that an all-female crew would be more of a publicity stunt than scientific research. Maybe the relevance of testing the impact of space on reproductive health will re-emerge on the research To Do list now that we're starting to consider manned deep-space missions and interplanetary travel.