  We report the detection of HCO+(5-4) emission from the Broad Absorption Line
(BAL) quasar APM08279+5255 at z=3.911 based on observations conducted at the
IRAM Plateau de Bure interferometer. This represents the first detection of
this molecular ion at such a high redshift. The inferred line luminosity,
uncorrected for lensing, is L'(HCO+)=(3.5+-0.6)x10^10 Kkms^-1pc^2. The HCO+
J=5-4 source position coincides within the errors with that reported from
previous HCN J=5-4 and high-J CO line observations of this quasar. The HCO+
line profile central velocity and width are consistent with those derived from
HCN. This result suggests that HCO+(5-4) emission comes roughly from the same
circumnuclear region probed by HCN. However, the HCN(5-4)/HCO+(5-4) intensity
ratio measured in APM08279+5255 is significantly larger than that predicted by
simple radiative transfer models, which assume collisional excitation and equal
molecular abundances. This could imply that the [HCN]/[HCO^+] abundance ratio
is particularly large in this source, or that the J=5 rotational levels are
predominantly excited by IR fluorescent radiation.
