The Gaussian-4 (G4) theory is a quantum chemical method for calculating molecular energies, improving upon the Gaussian-3 (G3) theory. The G4 theory incorporates five key modifications: (1) an extrapolation procedure to estimate the Hartree-Fock (HF) limit, (2) increased d-polarization sets with reoptimized exponents, (3) replacement of QCISD(T) with CCSD(T) for correlation treatment, (4) use of B3LYP density functional geometries and zero-point energies, and (5) two new higher-level correction parameters. These changes significantly improve the accuracy of energy predictions, reducing the average absolute deviation from experiment from 1.13 kcal/mol (G3) to 0.83 kcal/mol (G4). The largest improvements are observed for nonhydrogen species, with the deviation for 79 nonhydrogen systems decreasing from 2.10 kcal/mol (G3) to 1.13 kcal/mol (G4). The G4 theory is consistent with the Gaussian-n approach, combining well-defined ab initio calculations to achieve accurate energies without requiring extensive computational resources. The method is validated on the G3/05 test set, which includes 454 experimental energies. The G4 theory also includes a complete basis set model and addresses issues such as core-valence effects, relativistic effects, and spin-orbit corrections. The G4 theory is more accurate than the G3 theory for various types of energies, including enthalpies of formation, ionization potentials, electron affinities, and proton affinities. However, it still has some limitations, particularly for hydrogen-bonded complexes and certain nonhydrogen systems. The G4 theory is a significant advancement in quantum chemical methods, providing more accurate predictions for molecular energies.The Gaussian-4 (G4) theory is a quantum chemical method for calculating molecular energies, improving upon the Gaussian-3 (G3) theory. The G4 theory incorporates five key modifications: (1) an extrapolation procedure to estimate the Hartree-Fock (HF) limit, (2) increased d-polarization sets with reoptimized exponents, (3) replacement of QCISD(T) with CCSD(T) for correlation treatment, (4) use of B3LYP density functional geometries and zero-point energies, and (5) two new higher-level correction parameters. These changes significantly improve the accuracy of energy predictions, reducing the average absolute deviation from experiment from 1.13 kcal/mol (G3) to 0.83 kcal/mol (G4). The largest improvements are observed for nonhydrogen species, with the deviation for 79 nonhydrogen systems decreasing from 2.10 kcal/mol (G3) to 1.13 kcal/mol (G4). The G4 theory is consistent with the Gaussian-n approach, combining well-defined ab initio calculations to achieve accurate energies without requiring extensive computational resources. The method is validated on the G3/05 test set, which includes 454 experimental energies. The G4 theory also includes a complete basis set model and addresses issues such as core-valence effects, relativistic effects, and spin-orbit corrections. The G4 theory is more accurate than the G3 theory for various types of energies, including enthalpies of formation, ionization potentials, electron affinities, and proton affinities. However, it still has some limitations, particularly for hydrogen-bonded complexes and certain nonhydrogen systems. The G4 theory is a significant advancement in quantum chemical methods, providing more accurate predictions for molecular energies.