Kinetic temperature map of NGC 1333 IRAS 4 (left). During the fitting procedure all the seven H₂CO detected lines are taken into account for the positions with S/N> 3. The white contour shows the region where at least two lines were detected with S/N> 3. The kinetic temperature is higher toward the Class 0 objects IRAS 4A (~ 45 K) and IRAS 4B (~ 80 K), while it is significantly lower toward IRAS 4C (~ 12 K). We also note the compact emission toward IRAS 4-SWC (~ 15 K), which has been observed in channel maps of other molecules (e.g., CH₃OH, and SO, Fig. 11). The tau map of H₂CO 5_0,5–4_0,4 presents values in the optically thin regime throughout the cloud (middle; ~0.1−0.7). The best resulting χ² varies between ~1−5 in the region of the sources (right).