Wouldn't welding still leave 8 edges? In hex or modo you would join the 2 verts on each corner and that would create one edge on each side.

Also, Hex you should be able to select the 4 edges and fill it in and modo you would select the 4 verts and hit the P key.

I thought Daz had added some of hex into Carrara. Sorry if I have it wrong as I can't stand Carrara's modeling and use anything else.