Twofold reduced de Broglie wavelengths have been observed in two-photon interference experiments. With present technology it should be possible to observe a fourfold reduction when using a four-photon state from spontaneous parametric down-conversion. It is proposed to use a Mach–Zehnder set-up, which shows boson enhancement at the input beamsplitter and thus allows us to circumvent fourth-order non-linearities which would be necessary otherwise. The fourfold decrease in the observed de Broglie wavelength with perfect visibility is in principle achievable. This constitutes a reduction of the observed de Broglie wavelength below the wavelength of the generating source.