The combined intercalation of ions and solvents in layered structures—known as solvent co-intercalation—opens new pathways for designing advanced electrode materials. Here, we demonstrate electrochemically driven solvent co-intercalation in graphite electrodes using propylamine (PN) as the solvent and NaPF₆ as the conductive salt, yielding an interlayer spacing of 7.09 Å. Due to its low melting point and low viscosity, PN enables excellent low-temperature performance, with effective operation down to −30 °C. The poor cycle life is shown to be significantly improved by adding diglyme (2G) as second solvent, which is well-known to also undergo co-intercalation with Na+. The combined intercalation of PN and 2G results in the formation of quaternary graphite intercalation compounds (q-GICs). The properties of the electrolytes and electrodes are studied by galvanostatic cycling, X-ray diffraction, electrochemical impedance spectroscopy (EIS), operando microscopy, operando dilatometry, mass measurements, and theoretical modeling. Our results show that, in PN-based electrolytes, the number of co-intercalating solvent molecules varies strongly with the state of charge. At the beginning of the sodiation, more than 90 PN solvent molecules intercalate per Na+. This number decreases significantly as sodiation progresses, reaching an average of 3.6 PN molecules per Na+ in the fully sodiated state, corresponding to a stoichiometry of [Na:(PN)3.6]C28 (80 mA h g−1). Using PN also reduces the extent of electrode expansion and shrinkage during cycling, and it offers lower resistance and faster ion diffusion compared to pure 2G electrolytes. Overall, the combination of amines and ethers proves to be a promising strategy for tuning sodium storage properties in graphite electrodes.