The technical expertise of... buying four of the fanciest Nvidia GPU and plugging them into an off-the-shelf motherboard in an off-the-shelf chassis? A serious attempt at this kind of build would use the server variant of the RTX6000 and custom air ducts to cool them efficiently, but they packed four blower coolers like sardines so it no doubt sounds like a screaming jet engine under load.
The workstation versions are fine if you're running one or maybe two cards with an airflow gap between them, but if you pack four of them right next to each other then you're going to have a bad time when the fans get going.
