The microPET R4 scanner is a dedicated positron emission tomograph (PET) for studies of rodents. A number of scanner parameters such as spatial resolution, sensitivity, scatter, and count rate performance were determined in this work, which showed that the microPET R4 is a suitable PET scanner for small animals like mice and rats. In the center of the field of view (FOV) a maximal sensitivity of 43.66 cps/kBq for a centered point source was calculated from a measurement with a germanium-68 line source within an energy widow of 250-750 keV. A spatial resolution of 1.85 mm full-width at half-maximum (FWHM) in the axial direction and 1.66 mm FWHM in the transaxial direction was measured in the center with a 1-mm-diameter sodium-22 point source. Within the inner 20 mm of the FOV the volumetric resolution is better than 15.6 micro l, corresponding to a linear resolution of less than 2.5 mm in all three dimensions. Images of a high-resolution phantom and from mice and rat studies illustrate the good performance of the scanner. A maximal noise equivalent count rate (NECR) was reached at 174 kcps for a mouse phantom and at 93 kcps for a rat phantom (energy window 250-750 keV). Scatter fractions were measured between 0.30 and 0.42 for an energy window of 250-750 keV and phantom diameters similar to mice and rats. A comparison with the microPET P4 model for primates illustrates the gain in sensitivity due to a smaller detector ring diameter but also the changes in NECR.