17O NMR has been carried out in YBa2Cu4O8 with Tc = 74 K. The nuclear quadrupole tensors in the oxygen sites were determined and compared with those in other high-Tc cuprates. The hole densities in the oxygen and copper sites have been extracted from νz(O) and νQ(Cu) by assuming that the electric field gradient is dominated by the on-site holes. We propose that the hole distribution (the extent of covalency) between the planar oxygens and copper, which is changed by doping and pressure, is a hidden parameter determining the spin dynamics and the superconducting transition temperature Tc in high-Tc cuprates. It was found that 1/T1T and the Knight shift of the planar oxygens decrease with lowering temperature in the normal state. The nuclear relaxation rates of 17O and 63Cu in the CuO2 plane exhibit different behaviors at temperatures higher than ∼ 120 K, reflecting the different q-window of spin fluctuations, while both of them share the same T-dependence below ∼ 120 K down to 55 K. This scaling of 1/T1 between 17O and 63Cu, which is also seen in YBa2Cu3O7 (Tc = 92 K) and YBa2Cu3O6.63 (Tc = 60 K), seems to be essential for high-Tc superconductivity. © 1992.