  Very few debris discs have been imaged in scattered light at wavelengths
beyond 3 microns because the thermal emission from both the sky and the
telescope is generally too strong with respect to the faint emission of a
debris disc. We present here the first analysis of a high angular resolution
image of the disc of beta Pictoris at 3.8 microns. Our primary objective is to
probe the innermost parts of the beta Pictoris debris disc and describe its
morphology. We performed extensive forward modelling to correct for the biases
induced by angular differential imaging on extended objects and derive the
physical parameters of the disc. This work relies on a new analysis of seven
archival datasets of beta Pictoris observed with VLT/NaCo in the Lp band,
associated with disc forward modelling to correct for the biases induced by
that technique. The disc is detected above a 5 sigma level between 0.4" and
3.8". The two extensions have a similar brightness within error bars. We
confirm an asymmetry previously observed at larger distances from the star and
at shorter wavelengths: the isophotes are more widely spaced on the north-west
side (above the disc apparent midplane) than on the south-east side. This is
interpreted as a small inclination of the disc combined with anisotropic
scattering. Our best-fit model has an inclination of 86 degrees with an
anisotropic Henyey- Greenstein coefficient of 0.36. This interpretation is
supported by a new asymmetry detected in the disc: the disc is significantly
bowed towards the north-west within 3" (above the apparent midplane). We also
detect a possible new asymmetry within 1", but at this stage we cannot discern
between a real feature and an underlying speckle.
