How are the four fundamental forces related?
The four fundamental forces will be linked by a Theory of Everything (TOE).
Electromagnetism the the force which describes the interactions between charged particle. It is mediated by the photon.
Electricity and magnetism were originally thought to be separate forces until they were unified. Maxwell's equations describe electricity and magnetism.
Quantum Electrodynamics (QED) completed the picture by describing how electrons move in atoms.
The weak nuclear force is responsible for radioactivity and aspects of nuclear fission and fusion. It is propagated by the W and Z bosons. Typical weak interactions convert a proton into a neutron, a positron and an electron neutrino or a neutron into a proton, an electron and an electron anti-neutrino.
The electromagnetic force and the weak nuclear force were unified by the electro-weak theory.
The strong nuclear force is not really a force. The colour force also known as Quantum Chromodynamics (QCD) describes how quarks are bound together in protons, neutrons, mesons and other baryons. It is propagated by the gluon. The strong force is a residual effect of QCD operating at distance greater than the size of protons and neutrons.
Attempts are being made to create a Grand Unified Theory (GUT). Which unifies the electro-weak theory with QCD. A GUT is going to take some time to produce and verify.
This leaves gravity which actually isn't a force. It is the result of curved space time as described by Einstein's General Relativity.
A Theory of Everything (TOE) is required to unify a GUT and gravity. This is a long term goal.