The new Silhouette Cameo 4 has several new blades that can not be used in a Cameo 3. The Cameo 4 has 4 different adapters to assist with using the blades.

As you can see the adapters have a number 1 on them indicating which tool slot on the cameo 4 machine.
Cameo 4 sketch pen and third party tool
Cameo 4 premium blade and deep-cut
Cameo 4 white adapter is for 2 mm Kraft blade
Cameo 4 black adapter is for Blade aka ratchet blade and fabric blade