How is the theory of relativity related to the four fundamental forces?

The strong nuclear force is responsible for binding protons and neutrons into atomic nuclei. The weak nuclear force transforms protons into neutrons and neutrons into protons and is responsible for radioactive beta decay. Both the strong and weak nuclear forces are described by quantum theories. General Relativity and quantum mechanics are yet to be unified.

The electromagnetic force works at both quantum and larger distances. The outer electrons of heavier elements are moving at relativistic speeds. In fact the colour of gold is explained by general relativity.

The general theory of relativity is in fact a theory of gravity. Newton described gravity as a force. As gravity causes acceleration it can be described as a force as long as the masses and velocities of bodies are not too large.

Einstein's thought experiment said that a person in an elevator can't distinguish between gravity and acceleration. Relativity says that gravity is not a force it is the curvature of space time. So, for example a planet doesn't orbit its sun. Rather the planet follows a geodesic, which is the four dimensional equivalent of a straight line, in space time which is curved by the mass of the sun.