We have measured the spin susceptibility of the underdoped high temperature superconductor, YBa2Cu4O8 by Gd^{3+} electron spin resonance in single crystals and aligned powders at several magnetic fields between 3 and 15.4 T. At low temperatures and high fields, the spin susceptibility of the CuO2 planes is enhanced slightly in the $Bparallel c$ orientation with respect to the $Bperp c$ orientation. The enhancement in an applied field of 15.4 T ($approx 0.15 H_{c2}$) at 16 K (0.2 $T_c$) is approximately 10 percent of the susceptibility measured at $T_c$. Such a small magnitude suggests that the second critical field of superconductivity, $H_{c2}approx 100 T$, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high $T_c$ superconductors.
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