  We present a new spectrum extraction technique which employs optimal weights
for the spectral extraction, deblends the overlapping spectra, determines the
precise sky background, and takes into account correlations between errors
correctly for STIS slitless observations. We obtained roughly 250 optimally
extracted spectra in a deep STIS field as well as self-confirming redshift
measurements for these objects, including a galaxy at $z=6.68$. In addition, we
identified five isolated emission-line objects in the dispersed image that were
not accounted for by objects detected in the direct image. Assuming that these
are \lya\ emission line galaxies at high redshifts and adopting a simple
$\delta$-function galaxy luminosity function for these objects, we find, based
on a likelihood analysis, that \lya\ emission line galaxies at $\langle
z\rangle\approx 4.69$ may contribute at most 15\% of the star formation rate
density measured in Lyman break galaxies at $z\approx 4$.
