M4 doesn't have fusion connectors to make an Ultra chip. M5 Pro/Max however, are already built on fusion architecture from the beginning. So seems probable/likely they'll skip M4 Ultra (which they already confirmed not all chips will get an Ultra during the M4 lifecycle), and release an M5 Ultra later this year.
Analyzing the M3 Max on the microscope they concluded that it did not have fusion connectors, then they come out with the M3 Ultra... now it's the same story with the M4. Of course I hope in the M5 Ultra but the M4 Ultra possibility should not be excluded.