Lightweight design strategies and advanced energy applications call for highstrength Al alloys that can serve at 300-400 °C temperature range. However, the present commercial high-strength Al alloys are limited to low-temperature applications < ~150 °C, because the antagonism between high thermal stability (preferentially associated with slowdiffusing solutes) and large volume fraction (mostly derived from high-solubility and fastdiffusing solutes) innate to coherent nanoprecipitates remains unsolved. Here we demonstrate an interstitial solute stabilizing strategy to produce highly-stable coherent nanoprecipitates (termed V phase) in Sc-added Al-Cu-Mg-Ag alloys that have a strong coarsening-resistance up to ~ 400 ºC and simultaneously a volume fraction of ~ 1.8 vol.%. The assembling of slowdiffusing Sc and fast-diffusing Cu atoms into coherent V is triggered by a coherent ledgeaided in-situ phase transformation from early-formed Cu-rich  nanoprecipitates, with diffusion-dominated Sc uptake and self-organization into interstitially ordering. Stable V nanoprecipitates, with an inherited high density, enable the Al alloys to reach an unprecedent creep resistance as well as exceptional tensile strength (~ 100 MPa) at 400 ºC. We envisage that the ledge-mediated interaction between slow-and fast-diffusing atoms may pave the way for the stabilization of coherent nanoprecipitates toward advanced 400 ºC-level light alloys, which could be readily adapted to large-scale industrial production.