This is not the case. All 8 physical cores of the M1/2 can be utilized simultaneously, since each is a full physical core, independent of the others. The statement about only being able to use 4 at a time is true of threads, such as those found on SMT enabled processors that have core/thread counts like 4c/8t. In those systems, the threads share logic and decoding circuitry with each other, which would make your statement correct. You can see this in the M2 die shots. Both banks of 4 cores are in separate areas of the die. https://semianalysis.substack.com/p/apple-m2-die-shot-and-ar...
(edit): Granted 4 of the cores are lower powered "efficiency" cores, this is by no means equivalent to having just 4 cores
(edit): Granted 4 of the cores are lower powered "efficiency" cores, this is by no means equivalent to having just 4 cores