We performed high accuracy 63 Cu NQR spin-lattice relaxation and SQUID magnetization measurements on 16 O and 18 O exchanged YBa 2 Cu 4 O 8 to determine the isotope shift of the temperature of the opening of the spin gap, T ء , and the superconducting transition temperature, T c . The corresponding isotope exponents are a T ء 0.061͑8͒ and a Tc 0.056͑12͒ which are the same within the error bars and suggest a common origin for the superconducting and the spin gap. [S0031-9007(98)08085-5] 74.25.Nf, 74.62.Dh One of the central and heavily debated questions in high-temperature superconductivity research concerns the origin of the so-called pseudogap occurring in the normal state of underdoped superconductors [1]. The pseudogap refers to the transfer to higher energy of the density of low-energy excited states. One may ask whether the pseudogap is caused by superconducting correlations, which develop above T c , or whether it is an independent phenomenon. In nuclear magnetic resonance (NMR) and neutron scattering experiments, the pseudogap reveals itself as a spin gap. For instance, the Cu spin-lattice "relaxation rate per temperature unit," ͑T 1 T ͒ 21 , increases with falling temperature and reaches a maximum at T ء , which is a proper scale for the temperature dependence of the spin gap. For YBa 2 Cu 4 O 8 (Y124), the corresponding values are T c 81 K and T ء ഠ 150 K.Recently [2], we detected anomalies in the temperature dependence of several NMR and NQR (nuclear quadrupole resonance) quantities measured in the normal state of Y124, for instance in NQR frequencies, Knight shifts, line widths, and relaxation times. These anomalies, which occur around T y 180 K, are the signature of an electronic crossover which involves enhanced charge fluctuations in planes and chains. Because of the proximity of T y and T ء , we have argued that the spin gap effect in Y124 is caused by a transition due to a charge density wave (CDW) instability [3]. Using the t-J model and including electron-phonon interaction, we could, among others, explain the strong temperature dependence of the magnetic shift of the planar Cu nuclei, which we had measured previously [4], and we predicted a dependence of T ء on the isotope mass. Thus, corresponding measurements allow one to check the consistency of the CDW model.NMR͞NQR are techniques that can determine the pseudogap with a precision allowing one to establish whether an isotope effect is present or not. Y124, because of its well-defined oxygen stoichiometry and its negligible oxygen diffusion, is the ideal compound for such a study that requires experimental results not hampered by reproducibility problems. Therefore, the NQR study of the isotope effect on the spin gap in Y124 and its comparison with the superconducting gap is a unique experiment to explore the relation between the two gaps.In this Letter, we report a high-accuracy NQR study of the planar 63 Cu nuclei, supplemented by susceptibility measurements, on 16 O and 18 O exchanged Y124 samples which revealed the presenc...