  We present the faintest spectroscopically confirmed sample of redshift four
and five Lyman break galaxies to date. The sample is based on slitless grism
spectra of the Hubble Ultra Deep Field region from the GRAPES (Grism ACS
Program for Extragalactic Science) and PEARS (Probing Evolution and
Reionization Spectroscopically) projects, using the G800L grism on the HST
Advanced Camera for Surveys. We report here confirmations of 39 galaxies,
pre-selected as candidate Lyman break galaxies using photometric selection
criteria. We compare a "traditional" V-dropout selection to a more liberal one
(with V-i > 0.9), and find that the traditional criteria are about 64% complete
and 81% reliable. We also study the Lyman alpha emission properties of our
sample. We find that Lyman alpha emission is detected in about 1/4 of the
sample, and that our broad-band color selected sample includes 55% of
previously published line-selected Lyman alpha sources. Finally, we examine our
stacked 2D spectra. We demonstrate that strong, spatially extended (arcsecond
scale) Lyman alpha emission is not a generic property of these Lyman break
galaxies, but that a modest extension of the Lyman alpha photosphere (compared
to the starlight) may be present in those galaxies with prominent Lyman alpha
emission.
