  We present a spectral analysis of the UV-bright star Y453 in M4. Model fits
to the star's optical spectrum yield T_eff ~ 56,000 K. Fits to the star's FUV
spectrum, obtained with the Cosmic Origins Spectrograph (COS) on board the
Hubble Space Telescope, reveal it to be considerably hotter, with T_eff ~
72,000 K. We adopt T_eff = 72,000 +/- 2000 K and log g = 5.7 +/- 0.2 as our
best-fit parameters. Scaling the model spectrum to match the star's optical and
near-infrared magnitudes, we derive a mass M_* = 0.53 +/- 0.24 M_sun and
luminosity log L/L_sun = 2.84 +/- 0.05, consistent with the values expected of
an evolved star in a globular cluster. Comparing the star with post-horizontal
branch evolutionary tracks, we conclude that it most likely evolved from the
blue horizontal branch, departing the AGB before third dredge-up. It should
thus exhibit the abundance pattern (O-poor and Na-rich) characteristic of the
second-generation (SG) stars in M4. We derive the star's photospheric
abundances of He, C, N, O, Si, S, Ti, Cr, Fe, and Ni. CNO abundances are
roughly 0.25 dex greater than those of the cluster's SG stars, while the Si and
S abundances agree match the cluster values. Abundances of the iron-peak
elements (except for iron itself) are enhanced by 1 to 3 dex. Rather than
revealing the star's origin and evolution, this pattern reflects the combined
effects of diffusive and mechanical processes in the stellar atmosphere.
