To compare the signal beam with the incoming beams, we compare the images taken with a Ce-YAG screen located after the sample with and without the incoming beam open (that is, four holes or three holes opened in the spatial mask). a, Incoming beam opened in the signal direction (four holes opened in the mask). The photon energy was set at 876 eV. b, Same image as in panel a but scaled similarly as for the signal beam (see panel c) for comparison. c, XFWM signal detected with three holes opened in the spatial mask at the same photon energy, 876 eV. The signal beam profile is similar to the open incoming beam case. d, The XFWM signal vanishes when only two holes in the mask are opened, as expected from a FWM signal. The measurement in panel d was taken at 866.8 eV, around the maximum signal strength (see Fig. 1f). All measurements were done with the same gas pressure of 800 mbar. Each image is the average of 2,000 FEL shots.