  This paper presents ionization-corrected measurements of the carbon abundance
in intergalactic gas at 4.0 < z < 4.5, using spectra of three bright quasars
obtained with the MIKE spectrograph on Magellan. By measuring the CIV strength
in a sample of 131 discrete HI-selected quasar absorbers with
\rho/\bar{\rho}>1.6, we derive a median carbon abundance of [C/H]=-3.55, with
lognormal scatter of approximately ~0.8 dex. This median value is a factor of
two to three lower than similar measurements made at z~2.4 using CIV and OVI.
The strength of evolution is modestly dependent on the choice of UV background
spectrum used to make ionization corrections, although our detection of an
abundance evolution is generally robust with respect to this model uncertainty.
We present a framework for analyzing the effects of spatial fluctuations in the
UV ionizing background at frequencies relevant for CIV production. We also
explore the effects of reduced flux between 3-4 Rydbergs (as from HeII Lyman
series absorption) on our abundance estimates. At HeII line absorption levels
similar to published estimates the effects are very small, although a larger
optical depth could reduce the strength of the abundance evolution. Our results
imply that ~50% of the heavy elements seen in the IGM at z~2.4 were deposited
in the 1.3 Gyr between z~4.3 and z~2.4. The total implied mass flux of carbon
into the Lyman alpha forest would constitute ~30% of the IMF-weighted carbon
yield from known star forming populations over this period.
