The vision of the NanoSat Lab of the Universitat Politècnica de Catalunya (UPC BarcelonaTech) is the exploration and development of innovative applications and remote sensing techniques based on standard nano-satellite platforms (e.g. CubeSats). At the same time, we aim at developing our own platforms, while complying with the CubeSat standard. From the experience gained in our previous mission (3Cat-2, launched in August 2016), the NanoSat Lab is now continuing its labour with the 3Cat-4; whose mission is to demonstrate the capabilities of nano-satellites, and in particular those based on the 1U CubeSat, for challenging Earth observation using GNSS-R and L-Band microwave radiometry. Moreover, this mission has been selected by the European Space Agency (ESA) as part of the Fly Your Satellite! (FYS) 2017 programme and it is now under the supervision and guidance of ESA.