The Samsung Galaxy Z Fold4 has been rumored to feature a built-in S-Pen. Those reports look to have been dashed, however, as word on the grapevine indicates that the premium foldable will toe the same line as its predecessor, the Galaxy Z Fold3.
https://www.notebookcheck.net/Leaker-dashes-hopes-of-a-built-in-S-Pen-on-the-Samsung-Galaxy-Z-Fold4.614446.0.html