Mass-radius diagram for well-characterized planets (R < 4 R_⊕, M < 13 M_⊕) around M-dwarf stars (T_eff <4000 K), as retrieved from the Transiting Extrasolar Planet Catalogue (Southworth 2011). The masses and radii are known with a precision better than 30% and 10%, respectively. The theoretical composition models from Zeng et al. (2019) are displayed from bottom to top, with solid and dashed curves. The location of TOI-4438 b is marked with a red dot.