The superconducting transition temperatures ($T_{\mathrm{c}}$’s) of trilayer or quadruple-layer cuprates typically surpass those of single-layer or bilayer systems. However, the lack of direct electronic-structure and superconducting-gap measurements in optimal-$T_{\mathrm{c}}$ quadruple-layer cuprates has impeded a comprehensive understanding of the origin of the enhanced $T_{\mathrm{c}}$ in multilayer systems. In this Letter, using angle-resolved photoemission spectroscopy, we investigate the quadruple-layer cuprate $(\mathrm{Cu},\mathrm{C}){\mathrm{Ba}}_2{\mathrm{Ca}}_3{\mathrm{Cu}}_4{\mathrm{O}}_{11+\delta }$ (CuC-1234) with a high $T_{\mathrm{c}}$ of 110 K, and resolved distinct superconducting-gap behaviors between the inner ${\mathrm{CuO}}_2$ planes and outer ${\mathrm{CuO}}_2$ planes, in contrast to that reported for trilayer cuprates. Outer ${\mathrm{CuO}}_2$ planes develop their own superconducting gap and superconducting coherence peak at a temperature much lower than the $T_{\mathrm{c}}$ of the material, while the large pairing strength and phase coherence concurrently emerge at the underdoped inner ${\mathrm{CuO}}_2$ planes at $T_{\mathrm{c}}$. Our findings suggest that ${\mathrm{CuO}}_2$ planes free of apical oxygen can have significant contribution to superconductivity up to 110 K in multilayer cuprates, even at a doping level of 0.07 holes per Cu, a level that lies deep in the underdoped regime of single- and bilayer cuprates. These findings provide new insights into the origin of high $T_{\mathrm{c}}$ in multilayer cuprates.

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• Electronic structure
• Fermi surface
• Superconducting gap
• Superconducting order parameter
• Superconductivity
• Cuprates
• Angle-resolved photoemission spectroscopy