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 a signal-to-noise ratio 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 seven redshift bins. Combining galaxy-magnification cross-correlations and galaxy angular autocorrelations, 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 >7.5σ with a fixed σ₈ and is found to be consistent with the expected redshift dependence of the Wilkinson Microwave Anisotropy Probe 7 Λ cold dark matter cosmology.