Two-photon microscopy is a fluorescence imaging technique used to study living tissue for medical diagnosis and treatment monitoring. Compared to conventional one-photon microscopy, it can achieve greater depth and scatters less light, resulting in better spatial resolution. Correspondingly, four photon microscopy offers even greater spatial resolution, but it is restrictive due to the high excitation intensity of infrared lasers. Funded by the European Innovation Council, the 4for2 project proposes to merge the low excitation energies and high penetration depth of two-photon microscopy with the higher quality spatial resolution of four-photon microscopy. It will archive this by combining two-photon methods with negative photochromism and developing specially adapted fluorophores.