PETRA IV is the next big project at DESY, transforming the current synchrotron radiation source PETRA III into an ultra-low emittance storage ring. PETRA IV will deliver diffraction-limited synchrotron radiation up to 10 keV with pulse length in the range of 100 ps. This implies that the full beam can be focused to the diffraction limit without significant losses, thereby closing the mesoscopic resolution gap in x-ray analytics in the single-digit nanometer range. All x-ray analytical techniques, e. g., also inelastic and resonant scattering techniques, can go nano, delivering elemental, chemical, electronic, magnetic, and local structural contrast in x-ray microscopy. This will allow one to follow biological, chemical, and physical processes in situ on all relevant length scales and on time scales down to the nanosecond regime. The science case will be illustrated with various examples.