The separation of 1,3-butadiene (1,3-C4H6) and n-butene (n-C4H8) using ionic liquids (ILs) has received great attention in recent years, but designing an industrially attractive IL that combines large solubility and high selectivity remains challenging. In this work, we prepared several tributylethylphosphonium-based carboxylate ILs featuring large free volume and strong hydrogen bonding basicities through finely tuning the structures of cations and anions for the separation of C-4 hydrocarbons. These carboxylate ILs exhibited the highest mass solubility for 1,3-C4H6 yet reported (0.899 and 0.663 mmol/g in [P-4442][C17H35COO] and [P-4442][HCOO], respectively) and excellent 1,3-C4H6/n-C4H8 selectivity (1.83 in [P-4442][HCOO]) at 308.1 K and 100 kPa, which sets a new benchmark for 1,3-C4H6/n-C4H8 separation. Qualitative relationships between the free volume/basicity and their solubility were established. Results revealed that the free volume of unit mass and basicity were positively correlated with the 1,3-C4H6 solubility. The interaction between the ILs and gases was explored by quantum chemical calculations.