If space is infinite in every direction up,down,left,right and the universe is just a area of planets,stars...?

and Galaxy's in space then how can there be a fourth dimension. I saw a video with carl sagan describing two dimensions and third dimensions and then he desribed the fourth dimension. But if there were a fourth dimension wouldent this fourth dimension if it were outside our universe be in space. Because no matter how far outside our universe it is or large the fourth dimension is it would always be in space and because space is infinite in every direction how can it be another dimension. I heard scientists say the space inside a black hole is techniqly in another dimension. Thank you