In black: HDO lines observed towards IRAS 4A with HIFI, IRAM, and JCMT. The broad outflow component seen in the observations was subtracted. The continuum seen in the observations of the 894 GHz and 465 GHz lines refers to SSB data. In red: best-fit model obtained when adding an absorbing layer with an HDO column density of ~1.4 × 10¹³ cm^-2 to the structure. The best-fit inner abundance is 7.5 × 10^-9 and the best-fit outer abundance is 1.2 × 10^-11.