Multiphoton microscopy is a benchmark tool in biomedical research, used for the fluorescence imaging in cellular environments. This has important implications for disease diagnosis and the monitoring of therapy response. In conventional two-photon microscopy the fluorescence intensity of the employed molecular probe is proportional to the square of the excitation light intensity, implying that the fluorescence from the sample is confined around the focal point, yielding good spatial resolution. The spatial resolution can be dramatically improved by drawing on higher-order processes such as four-photon absorption. However, its practical implementation imposes major technical challenges, such as extreme laser intensities in the deep NIR. For this reason four-photon microscopy has so far attracted academic interest only. The present proposal addresses this issue and provides a multidisciplinary answer to the question: Can we develop a technique that offers spatial resolution of four-photon microscopy, but relies on two-photon absorption? This would combine the upsides of two-photon microscopy (low excitation energies provided by standard lasers at around 800 nm, high penetration depth in tissue) with the superior spatial resolution of four-photon microscopy. The result would be nothing less than a true paradigm shift in multiphoton microscopy. The key to tackle this highly ambitious task lies in the design of molecules that combine two mechanistically entangled two-photon processes (4for2) for the generation of a fluorescence output. This is possible by merging two-photon absorption, two-photon FRET-induced photoisomerization, and negative photochromism. Purposefully designed switchable fluorophores, that unify these photophysical assets, will be developed and their performance will be critically validated in a multi-angle spectroscopic work-flow, including the demonstration in application-relevant biological environments.