One example of a unified field theory that combines gravity, electromagnetism, and the weak nuclear force (but not the strong nuclear force, except in specific contexts) is the theory known as "electroweak theory."
Electroweak theory, developed in the 1960s, unifies the electromagnetic force and the weak nuclear force into a single electroweak force. It was proposed by physicists Sheldon Glashow, Abdus Salam, and Steven Weinberg. This theory treats the electromagnetic force and the weak nuclear force as different aspects of a more fundamental electroweak force.
In electroweak theory, the fundamental particles are described by a set of fields. The electromagnetic and weak forces are mediated by particles called gauge bosons. The photon is the gauge boson associated with the electromagnetic force, while the weak force is mediated by three particles: W+, W-, and Z bosons.
Gravity, however, is not included in electroweak theory. Gravity is described by Einstein's general theory of relativity, which is a theory of spacetime curvature and is not easily reconciled with quantum field theories like electroweak theory. Therefore, electroweak theory does not include a description of gravity.
In neutron stars, the strong nuclear force does play a role. Neutron stars are extremely dense objects composed mostly of neutrons. At such high densities, the strong nuclear force, which binds protons and neutrons within atomic nuclei, becomes relevant. However, the electroweak theory itself does not directly incorporate the strong nuclear force. The strong nuclear force is typically described by a separate theory called quantum chromodynamics (QCD).