  We present the discovery of a young L dwarf, WISE J231921.92+764544.4,
identified by comparing the Wide-field Infrared Survey Explorer (WISE) All-Sky
Catalog to the Two Micron All Sky Survey (2MASS). A medium-resolution optical
spectrum provides a spectral type of L4$\beta$, with a photometric distance
estimate of 26.1$\pm$4.4 pc. The red WISE $W1-W2$ color provides additional
evidence of youth, while the 2MASS $J-K_{\rm s}$ color does not. WISE
J231921.92+764544.4 is a candidate member of the young moving group Argus, with
the space motion and position of WISE J231921.92+764544.4 giving a probability
of 79% membership in Argus and a probability of 21% as a field object, based on
BANYAN II. WISE J231921.92+764544.4 has a mass of 12.1$\pm$0.4 M$_{\rm Jup}$
based on membership in Argus, within the planetary mass regime.
