Question

assuming the four fundamental forces are unifiable and arose from force, could a fifth force emerge in future time.
or can that cool stuff only happen pre-inflation

Answer 1

I'm pretty sure that would be highly unlikely, since that sort of thing seems to only happen in incredibly unstable situations like during Planck time. it's not even really clearly four fundamental forces since the weak nuclear and electromagnetic forces have been proposed to be the same under the "electro-weak theory". But I'm sure there are Theoretical physicists arguing about this as we speak.:)

Answer 2

well one force has split into two,
three times so far, why is this unlikely to happen again

Answer 3

My understanding is that during Planck time, quantum fluctuations had a large impact on the structure of the universe because it was so small at the time. Now quantum fluctuation is very small compared to the size of the universe, and so it only affects things on the subatomic level, which would not change the actual forces which control the whole universe simply because quantum fluctuation doesn't act on that large a scale. You make me wonder though, if maybe new forces pop up on the quantum scale just for a moment and then disappear, like those so-called "virtual particles". Frankly this is really above my head, I can only speculate based on what I know.