In black: HDO lines observed towards IRAS 4B with HIFI, IRAM, and APEX. 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 the absorbing layer used to model the IRAS 4A lines (N(HDO) ~ 1.4 × 10¹³ cm^-2). The best-fit inner abundance is 1 × 10^-8 and the best-fit outer abundance is 1.2 × 10^-10.