Did the four fundamental forces exist before the Big Bang? Or only after?
Yes all forces of nature existed nonpolarly in equilibrium
Out of equillibrated fourforcess arose unipolar concentrated four forceds quickly further and decoupling multi polar forces underlying creation
No, the four fundamental forces are defined by the universe.
First of all the term "before the big bang" is meaningless. People used to think that time was a continuum going from past to future. This model allows a before the big bang. Einstein showed that time is a dimension which is a fundamental part of the universe. Time as we know it simply didn't exist until the big bang.
The fundamental forces are also defined by the universe. Physicists are striving to find a Theory of Everything (TOE). In this model all of the fundamental forces were the same in the moments after the big bang. They later separated out into the forces we know today.
Gravity in particular is now known to be the effect of curved spacetime. So, gravity can't exist without a universe of spacetime which can be curved.
So, the four fundamental forces have only existed since the big bang. If there are other universes they could have different fundamental forces.