  We report on high-precision measurements of the temperature dependence of the
plane-^63Cu NQR line frequency nu_Q(Cu2) and the linewidth in normal and
superconducting ^16O and ^18O exchanged YBa_2Cu_4O_8. Whereas nu_Q(Cu2) passes
T_c very smoothly without a discontinuity either in value nor in slope, the
linewidth increases in the normal conducting phase down to T_c and starts to
decrease sharply in the superconducting phase to finally resume its
high-temperature value of the normal phase. There is a well discernible oxygen
isotope effect on the nu_Q(Cu2) temperature dependencies. The temperature
dependence of nu_Q(Cu2) is described by an empirical expression consisting of
two parts: one related to the thermal expansion of the lattice and the other
due to charge redistribution during the formation of new electronic structures
in the CuO_2 planes. From the fit to the experimental data we determine for the
conjectured formation of new electronic structures an energy scale Delta(^16O)
= 188.0(1.6) K and Delta(^18O) = 180.0(1.6) K. This results in a partial oxygen
isotope effect coefficient of 0.42(11) which is larger than both the
spin-pseudogap coefficient [0.061(8)] and the T_c coefficient [0.056(12)].
