According to a report by The Information, spotted by MacRumors, Apple was indeed developing a high-end M4 Extreme processor that would have included two high-performance M4 Ultra chips or four M4 Max chips, but those plans were reportedly shelved.
What's Known
The publication also says that this was likely done in order to prioritize the development of a server chip based on Apple Silicon. Those who follow Apple Silicon developments should probably be aware of a very similar situation with the M2 Extreme processor, which probably became the nail in the coffin of the Mac Pro's viability as a true monster among workstations.
However, if the M4 Extreme does somehow manage to see the light of day, it will most likely be a sizable 64-core processor, including a 160-core integrated graphics processor. The die size will be very large, which is not surprising, and the number of transistors will probably overshadow even server models.
The probability of the M4 Extreme's release is very vague at the moment and it remains to be seen whether Apple will return to this topic later.