Number of fans in MacBook Pro M4 cooling system

The base model M4 MacBook Pro has just one fan, the M4 Pro and M4 Max models have two fans (see here).

Nothing extra, just rearranged components. Basically, there's more space inside the chassis with only one fan, so they stick a few things there that are more tightly packed in the M4 Pro and M4 Max models. Click the link in my first post, scroll down to this picture and drag that red bar from side to side to see the differences in component layout.

if you are thinking you can add in another fan, Forget about it, you can't.

The mainboard will be a different shape than the models with two fans.

EDIT: and neuroanatomist has illustrated exactly what that difference is!