[N II]- and [S II]-BPT diagrams with line-flux ratio measurements for the four spectra of NGC 4156: archival SDSS 2004 (circle) and our TNG LR-B 2019 (square), LR-B 2022 (diamond), and VHR-R 2022 (blue marker) spectra. The VHR-R 2022 points have been obtained by combining the measurements of x-axis and y-axis ratios from the VHR-R and LR-B 2022 spectra, respectively. Solid and dashed black lines, separating between different ionisation regimes, are taken from Kewley et al. (2001, 2006) and Kauffmann et al. (2003). For clarity, we plot only the error bars relative to the TNG/LR-B 2022 measurements, comparable to the TNG/LR-B 2019 ones, while the SDSS 2004 (< 9%) and TNG/VHR-R (< 6%) are too small to be appreciated.