You simply cannot "rework" a CU on the fly. They have to be DESIGNED that way. The Tempest has been shown as a separate and different entity from the GPU. In fact, it's been stated that the Tempest engine is more related to the Cell processor and SPU's than it is a simple CU.
This is basically just another attempt to try and claim the PS5 has less power than stated and that it's "really" closer to that 9TF mark that so many loved to talk about early on.
So bottom line, GPU has 40 CU's with 4 disabled. Tempest engine is a separate part of the system. So is the SSD controller for Pete's sake before someone tries to claim that "really 4 of the 36 CU's are completely taken up controlling the SSD so the PS5 REALLY only has 32 CU's available for games!" or some crap like that.