  Using about 450,000 galaxies in the Deep Lens Survey, we present a detection
of the gravitational magnification of z > 4 Lyman Break Galaxies by massive
foreground galaxies with 0.4 < z < 1.0, grouped by redshift. The magnification
signal is detected at S/N greater than 20, and rigorous checks confirm that it
is not contaminated by any galaxy sample overlap in redshift. The inferred
galaxy mass profiles are consistent with earlier lensing analyses at lower
redshift. We then explore the tomographic lens magnification signal by
splitting our foreground galaxy sample into 7 redshift bins. Combining
galaxy-magnification cross-correlations and galaxy angular auto-correlations,
we develop a bias-independent estimator of the tomographic signal. As a
diagnostic of magnification tomography, the measurement of this estimator
rejects a flat dark matter dominated Universe at > 7.5{\sigma} with a fixed
\sigma_8 and is found to be consistent with the expected redshift-dependence of
the WMAP7 {\Lambda}CDM cosmology.
