The precise knowledge of neutrino flux and related uncertainties at the near and far detectors of the T2K experiment is crucial for extracting various neutrino oscillation parameters and neutrino cross-section measurements. The current Monte Carlo beam simulation framework, JNUBEAM, relies on the GEANT3 toolkits, which are no longer maintained. Additionally, it utilizes the FLUKA software to simulate hadronic production from interactions of the proton beam with the target. We aim to create a replacement framework solely based on well-established GEANT4 toolkits. Our new framework, namely G4JNUBEAM, uses GEANT4 available physics processes to consider primary proton interactions in the T2K target through to the decays of muons and hadrons, and subsequent production of neutrinos. We present simulation results for validation against NA61/SHINE data, neutrino flux predictions using G4JNUBEAM, and comparisons with the results obtained from the FLUKA+JNUBEAM simulations.