The issue, high glass-forming abilities (GFAs) in solute-absent multicomponent metallic glasses (MGs), has been investigated by systematic experimental measurements coupled with theoretical calculations in the Cu34Zr14Ti36Pd16 quaternary alloy. It is found that the synergetic contribution of some atomic- and cluster-level structural features leads to the high GFA in this quaternary composition. At the atomic scale, Pd and Ti addition decreases the size gaps between Cu and Zr atoms, and increases the atomic packing efficiency; at the cluster scale, there are more icosahedral or icosahedral-like local structures, and the cluster regularity is enhanced because of the optimization of both topological and chemical distributions in clusters. The present work provides an in-depth understanding of GFAs in multicomponent alloys, and will shed light on the development of more MGs with high GFAs.