Why are there four fundamental forces? How ere they deduced?
No one knows why!
This is really physics, not chemistry. There are understood to be four fundamental forces in the universe - electromagnetism, gravity, and the strong and weak nuclear forces. At the time of the big bang, there was most likely only one unified fundamental force but as the universe cooled the four forces we know of today were produced from this unified force.
Physicists have spent many years trying to work out how the forces are related to one another, with some degree of success, but there is much work still to be done.
As to why there are four forces, I don't think anyone knows. In the same way that no one knows why the speed of light has the value that it does, or why the charge on the electron is what it is. These factors appear to have been determined at the time of the big bang and there may be other universes where things are different and where there may be more or fewer fundamental forces.
You will probably get a better answer from a physicist!