You would have four participants in this group, each with their own key package signed by the nsec, and the clients reconcile them in the UI. In MLS each group message is only encrypted and uploaded once (which makes it scalable), and your four devices download the same event and decrypt it locally with the same key.

Great thanks! So in terms of what the other group members see with the naked eye there is only one me, yes, provided the clients in question are reconciling as they should?

Correct! This is one big feature of the MLS design, it's built for multi device users and large groups.