The fourth dimension is time. And our universe is getting longer in time (and so getting older). An explanation for this could be that a tetronian is adding time to it, making it longer. Like a trionian making a pillar longer.
The universe is also getting bigger (wider). The tetronian could be adding material to it, like making a concrete pillar stronger.
This would be interesting. Tetronians building houses out of universes? Why not.