Coherent nonlinear X-ray four-photon interaction with core-shell electrons - Ana Sofia Morillo-Candas; Sven Augustin; Eduard Prat; Antoine Sarracini; Jonas Knurr; Serhane Zerdane; Zhibin Sun; Ningchen Yang; Marc Rebholz; Hankai Zhang; Yunpei Deng; Xinhua Xie; et al. (Nature, 2026)