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Soyuz MS-04 has the distinction of being the first manned launch of 2017, and the first (at least in a while?) to bring only two, rather than three crew members to the ISS.
Is there an official reason? Is there also a subtext?
below: "Soyuz MS-04 is being prepared for vacuum testing." from here. Credit: RKK Energia
Beginning with the next Soyuz launch, in March 2017, Russia is cutting one cosmonaut post from the station crew and doesn't plan to restore the job until a new module is launched, in 2018.
Cosmonauts typically comprise half the station's six-member crew, but "right now, we don't need it," Sergei Krikalev, a former cosmonaut who now oversees human spaceflight for the Russian space agency, Roscosmos, said in an interview at the International Astronautical Congress, which was held in late September in Guadalajara, Mexico.
The subtext is budget cuts, I believe.
In addition to Hobbes' answer, I may add this point.
I do not know if this is a cause or a consequence, but the Soyuz MS-04 will bring back to earth, in addition of the two crew members sent upthere, Peggy Whitson. She lift off with Soyuz MS-03 to be part of Expedition 50 but did not return back to Earth with her two fellow travellers, who landed safely June 2, 2017.
This is due to an agreement between NASA and the Russian space agency Roscosmos, to adds three months to record-breaking mission.
Obviously, she will need an extraseat to land back on Earth, and this seat will be in Soyuz MS-04.
