  We report on i-band snapshot observations of 157 Sloan Digital Sky Survey
(SDSS) quasars at 4<z<5.4 using the Advanced Camera for Surveys on the Hubble
Space Telescope (HST) to search for evidence of gravitational lensing of these
sources. None of the quasars appear to be strongly lensed and multiply imaged
at the angular resolution (~0.1") and sensitivity of HST. The non-detection of
strong lensing in these systems constrains the z=4-5 luminosity function to an
intrinsic slope of beta>-3.8 (3 sigma), assuming a break in the quasar
luminosity function at M_1450^star=-24.5. This constraint is considerably
stronger than the limit of beta>-4.63 obtained from the absence of lensing in
four z>5.7 quasars. Such constraints are important for our understanding of the
true space density of high-redshift quasars and the ionization state of the
early universe.
