Because the pcie is x8 electrical, the adapters in the market that relies on bifurcation would be only be able to do 2 M.2 at x4. So it is only possible with adapters that include a bridge, and those will likely be super expensive or sacrifice perf.
If instead you build one adapter you should be able to do 4 M.2 slots each one x2 4.0 PCIE, if it is wired correctly for this case, no bridge and nothing fancy requiered, just the M.2 slots and some supporting electronics to handle power delivery.
8 M.2 @ x1 4.0 might be possible too, it all depends on the bifurcation capabilities of the SoC.