Left: spatial distribution of galaxies with photometric redshift intervals consistent with z = 3.95. The dashed box denotes the 1′ × 1′(∼2 cMpc × 2 cMpc) region of the galaxy overdensity, LH-SBC3. The dots are color-coded by their stellar masses. Right: zoom-in RGB image (red: IRAC2; green: K; blue: i) of LH-SBC3. The white solid contours denote the 3 mm continuum, starting at 2σ in steps of 1σ. The four CO-confirmed members are labelled with their IDs. Note: LH-SBC3.a and d are very close. The cyan dashed circles are the IRAC-color selected priors. The priors labelled with a question mark could come from another structure at z = 3.25 or z = 2.19, see discussion in Sect. 3. The dotted white contours show 500 μm flux at 20, 40, 60 mJy beam⁻¹ levels.