you can do this in the main game, hop on the empty "nintendo" sign at the stage near the start and do a blj off the vent, make sure to switch character or explode if your going too fast then land on the door thats below the building, as for the 4th floor you have to blj in the outside of rec room
In rec room you have to spam jump on top of the beeg bob-omb to go into the floor that is out of bounds, then infront of you theres a wall, that you need to blj in it to reach the forth floor