As we get further into the film, we learn that even a single crew member is capable of undertaking Plan B, operating the artificial wombs on the Endurance and managing the population bomb cryogenics. Frankly it's overkill to even take four crewmen when...
there was never any real hope of operating Plan A.
On top of that, the ship is obvious designed to carry only a very small crew. Aside from the living module, the cargo areas and the propulsion units, there's barely enough space to fit four cryo-pods. They could probably have squeezed another couple in but when every breath counts, there's very little mileage in adding extra people merely for the sake of it.
The official novelisation makes the living situation a little clearer;
“Hey, we agreed,” she said. “Ninety percent.” With that she went to
her own cryo-bed. Cooper returned his gaze to the infinite space
outside of the ship.
“Don’t stay up too late,” Brand instructed. “We can’t spare the
“Hey,” Cooper objected with mock chagrin. “I’ve been waiting a long
time to be up here.” “You are literally wasting your breath,” she
As they boarded the Endurance, it became clear that it wasn’t as roomy
as it looked from the outside. Part of this was because two-thirds of
each of the modules was taken up by storage. The floors, the
walls—almost every surface was composed of hatches of various sizes.
On a deep-space vessel, there could be no wasted space—not even one
the size of a matchbox.