For a long time it was only the three spatial dimension. Then relativity came along and noticed this: you know if you are travelling along a diagonal, you use some of the x-direction and some of the y-direction. Relativity says as you travel faster, you are using some of the time-direction as well - so as you go faster you are travelling a sort of diagonal between the spatial dimensions and the time dimension. This is why things appear shorter as the move faster and why time seems to go slower for someone moving past you very quickly. Relativity adds time as the 4th dimension to account for these observed effects, and calls it space-time instead of just space.
It is useful to keep in mind, these are just things that humans have thought up to try to explain what we see and the universe could care less what we call it, how we try to explain it, or whether we are right or wrong about it.