X670E Taichi bifurcation question

Bifurcation of a slot only affects that slot.
8x4x4 means that the 16 lanes will be split into one 8x and two 4x lanes.
To make use of this, you need some form of an adapter, or a PCIe card with separate devices on board.

An adapter example is a 16x adapter that gives you two 8x slots.

A card example is a 16x card with four m.2 slots onboard, each using 4 lanes.
If you were to use such an adapter on a slot that does 8x4x4x, you won't be able to use all 4 m.2 slots because one of them won't be independent. You'll end up using m.2 slot 0 (using 4 lanes out of 8), and then slots 2 and 3 (4 lanes each). You won't be able to use slot 1 because that will be wired to second half of the 8 lanes that are not independent.