The OP’s scenario would approach a foursome only if the two couples are acknowledging their awareness of the situation to each other and communicating with each other in some way.
Or could be someone who actually experienced once what the OP describes and whose partner also experienced the same thing, they both experienced it together, and were aware that they were intentionally exchanging sonic sex with the other couple downstairs, who, they both felt, were also aware of it and going along with it. Sleeping in late on an otherwise placid Saturday morning. They shared a shared experience of social networking ancient-style. And found it oddly sexy and thrilling for taking place on the edge of maybe beyond what’s proper…
Right. You get what I mean.