  PAON4 is an L-band (1250-1500 MHz) small interferometer operating in transit
mode deployed at the Nan\c{c}ay observatory in France, designed as a prototype
instrument for Intensity Mapping. It features four 5~meter diameter dishes in a
compact triangular configuration, with a total geometric collecting area of
$\sim75 \mathrm{m^2}$, and equipped with dual polarization receivers. A total
of 36 visibilities are computed from the 8 independent RF signals by the
software correlator over the full 250~MHz RF band. The array operates in
transit mode, with the dishes pointed toward a fixed declination, while the sky
drifts across the instrument. Sky maps for each frequency channel are then
reconstructed by combining the time-dependent visibilities from the different
baselines observed at different declinations. This paper presents an overview
of the PAON4 instrument design and goals, as a prototype for dish arrays to map
the Large Scale Structure in radio, using intensity mapping of the atomic
hydrogen $21~\mathrm{cm}$ line. We operated PAON4 over several years and use
data from observations in different periods to assess the array performance. We
present preliminary analysis of a large fraction of this data and discuss
crucial issues for this type of instrument, such as the calibration strategy,
instrument response stability, and noise behaviour.
