  We present the first comprehensive evolutionary analysis of the rest-frame UV
spectroscopic properties of star-forming galaxies at z ~ 2-4. We match samples
at different redshifts in UV luminosity and stellar mass, and perform
systematic measurements of spectral features and stellar population modeling.
By creating composite spectra grouped according to Ly$\alpha$ equivalent width
(EW), and various galaxy properties, we study the evolutionary trends among
Ly$\alpha$, low- and high-ionization interstellar (LIS and HIS) absorption
features, and integrated galaxy properties. We also examine the redshift
evolution of Ly$\alpha$ and LIS absorption kinematics, and fine-structure
emission EWs. The connections among the strengths of Ly$\alpha$, LIS lines, and
dust extinction are redshift-independent, as is the decoupling of Ly$\alpha$
and HIS line strengths, and the bulk outflow kinematics as traced by LIS lines.
Stronger Ly$\alpha$ emission is observed at higher redshift at fixed UV
luminosity, stellar mass, SFR, and age. Much of this variation in average
Ly$\alpha$ strength with redshift, and the variation in Ly$\alpha$ strength at
fixed redshift, can be explained in terms of variations in neutral gas covering
fraction and/or dust content in the ISM and CGM. However, based on the
connection between Ly$\alpha$ and CIII] emission strengths, we additionally
find evidence for variations in the intrinsic production rate of Ly$\alpha$
photons at the highest Ly$\alpha$ EWs. The challenge now is to understand the
observed evolution in neutral gas covering fraction and dust extinction within
a coherent model for galaxy formation, and make robust predictions for the
escape of ionizing radiation at z > 6.
