Summary of our main results. The top panel shows the retrieved water mixing ratios and elemental abundances of carbon and oxygen for all four HR 8799 exoplanets. For HR 8799d and e, we show the water abundance in chemical equilibrium at 1 bar (represented by the blue stars). For C/H and O/H, we also show the corresponding values of the HR 8799 star (horizontal dashed lines). The bottom panel shows the exoplanetary elemental abundances normalized to their stellar values with the dashed line denoting parity.