  We present the discovery of three late type (>T4) brown dwarfs, including a
probable Y dwarf, in the WFC3 Infrared Spectroscopic Parallels (WISP) Survey.
We use the G141 grism spectra to determine the spectral types of the dwarfs and
derive distance estimates based on a comparison with nearby T dwarfs with known
parallaxes. These are the most distant spectroscopically confirmed T/Y dwarfs,
with the farthest at an estimated distance of ~400 pc. We compare the number of
cold dwarfs found in the WISP survey with simulations of the brown dwarf mass
function. The number found is generally consistent with an initial stellar mass
function dN/dM \propto M^{-\alpha} with \alpha = 0.0--0.5, although the
identification of a Y dwarf is somewhat surprising and may be indicative of
either a flatter absolute magnitude/spectral type relation than previously
reported or an upturn in the number of very late type brown dwarfs in the
observed volume.
