Here we use high-precision carbon isotope data (delta C-13-CO2) to show atmospheric CO2 during Marine Isotope Stage 4 (MIS 4, similar to 70.5-59 ka) was controlled by a succession of millennial-scale processes. Enriched delta C-13-CO2 during peak glaciation suggests increased ocean carbon storage. Variations in delta C-13-CO2 in early MIS 4 suggest multiple processes were active during CO2 drawdown, potentially including decreased land carbon and decreased Southern Ocean air-sea gas exchange superposed on increased ocean carbon storage. CO2 remained low during MIS 4 while delta C-13-CO2 fluctuations suggest changes in Southern Ocean and North Atlantic air-sea gas exchange. A 7 ppm increase in CO2 at the onset of Dansgaard-Oeschger event 19 (72.1 ka) and 27 ppm increase in CO2 during late MIS 4 (Heinrich Stadial 6, similar to 63.5-60 ka) involved additions of isotopically light carbon to the atmosphere. The terrestrial biosphere and Southern Ocean air-sea gas exchange are possible sources, with the latter event also involving decreased ocean carbon storage.