so i'm moving the toilet and shower to the opposite side of the bathroom a whole 4 feet away from where it originally was. i'm curious if i can use the vent pipe i already had in place or do i need to install a new one
No, it would be foolish to do so, I'm assuming it goes through the roof. You would need to make a hole, patch a hole, install the flashing and roof it in, and probably you wouldn't know how to do that. If you can just run the elbows and glue the joints in existing vent you are good to go.