SpaceX CRS-4, also known as SpX-4, was a cargo resupply mission to the International Space Station, contracted to NASA, which was launched on 21 September 2014 and arrived at the space station on 23 September 2014. It was the sixth flight for SpaceX's uncrewed Dragoncargo spacecraft, and the fourth SpaceX operational mission contracted to NASA under a Commercial Resupply Services contract. The mission brought equipment and supplies to the space station, including the first 3D printer to be tested in space, a device to measure wind speed on Earth, and small satellites to be launched from the station. It also brought 20 mice for long-term research aboard the ISS.
NASA contracted for the CRS-4 mission and therefore determined the primary payload, date/time of launch, and target orbital parameters. The payload consisted of 4,885 pounds of cargo, including 1,380 pounds of crew supplies. The cargo included the ISS-RapidScat, a Scatterometer designed to support weather forecasting by bouncing microwaves off the ocean’s surface to measure wind speed, which was launched as an external payload to be attached on the end of the station's Columbus laboratory. CRS-4 also includes the Space Station Integrated Kinetic Launcher for Orbital Payload Systems (SSIKLOPS), which will provide still another means to release other small satellites from the ISS. In addition, CRS-4 carried a new permanent life science research facility to the station: the Bone Densitometer (BD) payload, developed by Techshot, which provides a bone density scanning capability on ISS for utilization by NASA and the Center for the Advancement of Science in Space (CASIS). The system measures bone mineral density (and lean and fat tissue) in mice using Dual-Energy X-ray Absorptiometry (DEXA). 
SpaceX has primary control over manifesting, scheduling and loading secondary payloads. However there are certain restrictions included in their contract with NASA that preclude specified hazards on the secondary payloads, and also require contract-specified probabilities of success and safety margins for any SpaceX reboosts of the secondary satellites once the Falcon 9 second stage has achieved its initial low-Earth orbit (LEO).
The CRS-4 mission carried the 3D Printing in Zero-G Experiment to the ISS, as well as a small satellite as secondary payload that will be deployed from the ISS: SPINSAT. It also brought 20 mice for long-term physiological research in space.
The 3D Printing in Zero-G Experiment will demonstrate the use of 3D printing technology in space. 3D printing works by the process of extruding streams of heated material (plastic, metal, etc.) and building a three-dimensional structure layer-upon-layer. The 3D Printing in Zero-G Experiment will test the 3D printer specifically designed for microgravity, by Made In Space, Inc., of Mountain View, California. Made In Space’s customized 3D printer will be the first device to manufacture parts away from planet Earth. The 3D Printing in Zero-G Experiment will validate the capability of additive manufacturing in zero-gravity. This experiment on the International Space Station is the first step towards establishing an on-demand machine shop in space, a critical enabling component for deep-space crewed missions and in-space manufacturing.
SPINSAT is a technology demonstrator for electric solid propellant (ESP) thrusters from Digital Solid State Propulsion (DSSP). DSSP's technology utilizes electric propulsion to enable small satellites to make orbital maneuvers that have generally not been possible in the very small, mass-constrained satellites such as CubeSats and nanosats. This will be DSSP's first flight and will be deployed from the Kibo module airlock. NASA safety experts approved the mission—which by its nature must start with the satellite inside the habitable volume of the ISS—because the satellite's 12 thruster-clusters burn an inert solid fuel, and then only when an electric charge is passed across it.
Payloads are separated by bullets ( · ), launches by pipes ( | ). Manned flights are indicated in bold text. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in (brackets).