ALMA has revolutionised our ability to characterise the cold gas and dust in distant galaxies through the 158 micron [CII] cooling line and dust continuum. With deep HST imaging and the successful launch of JWST, we are entering an era where stars and gas can now be studied in unprecedented detail. The few pilot studies of spatially resolved galaxies at high-z demonstrate the complexity of their ISM morphology, and reveal variations in the [CII]/TIR ratio driven by local processes. The multi-phase origin of the [CII] line further complicates its use as a SFR tracer, and the community is unable to reach a consensus on whether the local SFR-[CII] relation continues at high-z. We propose to exploit the new ALMA Large Program CRISTAL studying nineteen z~4-5 main-sequence galaxies in [CII] and dust continuum emission on kilo parsec scales. CRISTAL will enable the next frontier of multi-wavelength resolved galaxy studies, combining ALMA, HST, and imminently JWST observations to understand the connections between gas, dust, stars, and star formation. In this FWO project, we focus on studying the ISM morphology, resolving local variations in the [CII]/TIR ratio and reviewing the applicability of [CII] as a SFR indicator at high-z. Finally, we will present new calibrations of the high-z SFR-[CII] relation — corrected for the effect of [CII] deficits — to the community, which are desperately needed to exploit high-z observations of the workhorse [CII] line to its full potential.