<span>Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key for elucidating the ocean’s role in modulating atmospheric CO<sub>2</sub> changes on millennial and orbital timescales. Marine Isotope Stage (MIS) 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO<sub>2</sub> levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO<sub>2</sub> drop of ~40 ppmv, likely linked to changes in ocean circulation, and includes several millennial climatic events, for example Heinrich Stadial (HS) 6. Although previous proxy-based studies have suggested a weakened Atlantic overturning during MIS 4, and particularly HS 6, basin wide changes in circulation remain poorly constrained. Here, we present high-resolution deep-water hydrography reconstructions from the mid-depth Iberian Margin (core MD01-2444, ~2.65 km water depth), based on benthic foraminiferal Mg/Ca-derived deep-water temperature (T<sub>dw</sub>), benthic </span><span>δ</span><sup><span>18</span></sup><span>O, and associated δ</span><sup><span>18</span></sup><span>O<sub>dw</sub> records, alongside SS</span><span></span><span>- based flow-speed record.</span><span> </span><span>Our data reveal three distinct deep-water hydrographic ‘regimes’ at the Iberian Margin: 1) warm “interglacial-like” mode during MIS 5a (including Greenland stadials C19 and C20); 2) a colder glacial circulation mode during early MIS 4 (pre- HS 6); and 3) a “Heinrich” circulation mode during HS 6. </span><span>A stronger influence of colder southern-sourced waters is inferred at the Iberian Margin throughout MIS 4 and intensifies during HS 6, whereas MIS 5a stadials are characterised by a pronounced subsurface warming. The warm stadials C19 and C20 were likely driven by a southward displacement of convection sites in the North Atlantic and appear to be unique to MIS 5a as they are not observed during MIS 3 Dansgaard-Oeschger events at this site. We also find evidence of “millennial-type” variability and, similar to recent studies for MIS 2, a persistent contribution of northern sourced waters during MIS 4 (pre- HS 6). The contrasting impacts of similar millennial-scale DO-type variability on North Atlantic deep-water hydrography and circulation strength during MIS 3, 4, and 5 highlight the strong dependence on the background climate state. </span><span></span>