Retrieval using mock JWST emission spectra of a TrES-4b-like planet. Panel a, top: synthetic observations (black circles) and best-fit spectrum (red solid line). Panel a, bottom: residuals between the best-fit spectrum and mock observations, in units of the observational errors. Panel b: emission contribution function, calculated using Eq. (4). Pressures higher than 4 bar cannot be probed. Panel c: confidence envelopes of the retrieved P-T profiles (light gray to dark blue) and input P-T profile (red solid line). The last 600 000 samples were used for making this plot. The probing limit, inferred from panel b, is shown as a horizontal black dashed line. Below this altitude the P-T profile cannot be probed, all posterior envelopes are determined by the analytic form of the P-T function (Eq. (15)). Panel d: 2-d posterior plot of the retrieved log-mass fractions, input values are shown as red dashed lines. The last 700 000 samples were used for making this plot.