| Summary: | <jats:p> The two-spin solid effect (2SSE) is one of the established continuous wave dynamic nuclear polarization mechanisms that enables enhancement of nuclear magnetic resonance signals. It functions via a state-mixing mechanism that mediates the excitation of forbidden transitions in an electron–nuclear spin system. Specifically, microwave irradiation at frequencies ω<jats:sub>μ w</jats:sub> ∼ ω<jats:sub>0S</jats:sub> ± ω<jats:sub>0I</jats:sub>, where ω<jats:sub>0S</jats:sub> and ω<jats:sub>0I</jats:sub> are electron and nuclear Larmor frequencies, respectively, yields enhanced nuclear spin polarization. Following the recent rediscovery of the three-spin solid effect (3SSE) [Tan et al., Sci. Adv. 5, eaax2743 (2019)], where the matching condition is given by ω<jats:sub>μ w</jats:sub> = ω<jats:sub>0S</jats:sub> ± 2 ω<jats:sub>0I</jats:sub>, we report here the first direct observation of the four-spin solid effect (4SSE) at ω<jats:sub>μ w</jats:sub> = ω<jats:sub>0S</jats:sub> ± 3 ω<jats:sub>0I</jats:sub>. The forbidden double- and quadruple-quantum transitions were observed in samples containing trityl radicals dispersed in a glycerol–water mixture at 0.35 T/15 MHz/9.8 GHz and 80 K. We present a derivation of the 4SSE effective Hamiltonian, matching conditions, and transition probabilities. Finally, we show that the experimental observations agree with the results from numerical simulations and analytical theory. </jats:p>
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