The Compute module 4 is simultaneously stripped down and beefed up. For example, there are no HDMI ports, USB ports, camera ports, or any of the usual bits to plug into. Instead there's a pair of high-density connectors that can plug into a carrier board. While Raspberry Pi have released a carrier board (the Raspberry Pi Compute Module 4 Input/Output board) that breaks-out all the features of the Compute Module 4, we expect many companies and makers to design their own. To make this easy, Raspberry Pi have released the KiCAD files for the IO board so you can tweak the designs however you like.
Raspberry Pi CEO Eben Upton and Compute Module 4 designer Domonic Plunkett discuss the new products
For an in depth look at the new board, and how it compares to older Compute Modules, take a look at the MagPi's excellent analysis.