  We used the Atacama Large (sub-)Millimeter Array (ALMA) and the IRAM Plateau
de Bure Interferometer (PdBI) to image, with an angular resolution of 0.5$''$
(120 au) and 1$''$ (235 au), respectively, the emission from 11 different
organic molecules in the protostellar binary NGC1333 IRAS 4A. We clearly
disentangled A1 and A2, the two protostellar cores present. For the first time,
we were able to derive the column densities and fractional abundances
simultaneously for the two objects, allowing us to analyse the chemical
differences between them. Molecular emission from organic molecules is
concentrated exclusively in A2 even though A1 is the strongest continuum
emitter. The protostellar core A2 displays typical hot corino abundances and
its deconvolved size is 70 au. In contrast, the upper limits we placed on
molecular abundances for A1 are extremely low, lying about one order of
magnitude below prestellar values. The difference in the amount of organic
molecules present in A1 and A2 ranges between one and two orders of magnitude.
Our results suggest that the optical depth of dust emission at these
wavelengths is unlikely to be sufficiently high to completely hide a hot corino
in A1 similar in size to that in A2. Thus, the significant contrast in
molecular richness found between the two sources is most probably real. We
estimate that the size of a hypothetical hot corino in A1 should be less than
12 au. Our results favour a scenario in which the protostar in A2 is either
more massive and/or subject to a higher accretion rate than A1, as a result of
inhomogeneous fragmentation of the parental molecular clump. This naturally
explains the smaller current envelope mass in A2 with respect to A1 along with
its molecular richness.
