  We present the discovery of a wide (67 AU) substellar companion to the nearby
(21 pc) young solar-metallicity M1 dwarf CD-35 2722, a member of the ~100 Myr
AB Doradus association. Two epochs of astrometry from the NICI Planet-Finding
Campaign confirm that CD-35 2722 B is physically associated with the primary
star. Near-IR spectra indicate a spectral type of L4\pm1 with a moderately low
surface gravity, making it one of the coolest young companions found to date.
The absorption lines and near-IR continuum shape of CD-35 2722 B agree
especially well the dusty field L4.5 dwarf 2MASS J22244381-0158521, while the
near-IR colors and absolute magnitudes match those of the 5 Myr old L4
planetary-mass companion, 1RXS J160929.1-210524 b. Overall, CD-35 2722 B
appears to be an intermediate-age benchmark for L-dwarfs, with a less peaked
H-band continuum than the youngest objects and near-IR absorption lines
comparable to field objects. We fit Ames-Dusty model atmospheres to the near-IR
spectra and find T=1700-1900 K and log(g) =4.5\pm0.5. The spectra also show
that the radial velocities of components A and B agree to within \pm10 km/s,
further confirming their physical association. Using the age and bolometric
luminosity of CD-35 2722 B, we derive a mass of 31\pm8 Mjup from the Lyon/Dusty
evolutionary models. Altogether, young late-M to mid-L type companions appear
to be over-luminous for their near-IR spectral type compared to field objects,
in contrast to the under-luminosity of young late-L and early-T dwarfs.
