We study the properties of the interstellar medium in the interacting system BR1202-0725 at z = 4.7 via its [N ii] and [C ii] fine-structure line emission. This system consists of a QSO, a sub-mm galaxy (SMG), and two Lyα emitters (LAEs). Such a diversity in galaxy properties makes BR1202-0725 a unique laboratory of star formation and galaxy evolution at high redshift. We present ionized nitrogen ([N ii] 205 μm) observations of this system, obtained with the IRAM Plateau de Bure Interferometer. We find no [N ii] emission at the quasar location, but tentative [N ii] line detections associated with the SMG and one of the LAEs. Together with available ionized carbon ([C ii] 158 μm) Atacama Large Millimeter Array observations of this system, we find the following: the [C ii]/[N ii] luminosity ratio is >5.5 for the QSO and the SMG, but it is as low as ∼2 in the LAE, suggesting that, in this source, most of the [C ii] emission is associated with the ionized medium (H ii regions) rather than the neutral one (photon-dominated regions). This study demonstrates the importance of combined studies of multiple fine-structure lines in order to pin down the physical properties of the interstellar medium in distant galaxies.