Thanks guys. I've spoken to Duravit and their cus. support tells me their wall-hung carriers are available for 2x4 or 2x6 framing, just like Toto.....but their in-wall tanks for the above floor-standing, rear-outlet Starck 2 model is only for a 2x6 wall. Stymied at every turn!
That doesn't make a lot of sense to me, so I've asked them to search for a floor-standing, rear-outlet model that can have a 2x4 in-wall tank....hopefully they find something for me.