The answer is rather simple, really: Nothing has changed compared to the Galaxy Z Fold 3 when it comes to stylus support. So yes, the Z Fold 4 can be used with the S Pen, or rather the S Pen Fold Edition (bought separately), which has a unique tip designed to work with the foldable display. And no, you still need to keep the stylus separate from the phone, as there's no built-in slot for it.
Galaxy Z Fold 4 supports S Pen input on the folding (inner) display
Furthermore, the cover screen still doesn't support S Pen input. That probably won't disappoint too many people, as the cover screen is once again too narrow to provide a comfortable experience in S Pen use cases, such as drawing and taking notes.
And, again, since the Fold Edition S Pen has a unique tip designed to only work with the inner screen (which is made of plastic and has an ultra-thin layer of glass on top), adding stylus support to the outer screen would have just confused things, as users would have to carry around two different styluses if they wish to have S Pen input on both Galaxy Z Fold 4 displays.
So there you have it: The Galaxy Z Fold 4 supports S Pen input on the foldable display, doesn't have a slot built-in to carry the S Pen (although Samsung will provide a case that has a nice little pocket for the stylus), and the S Pen Fold Edition is a separate purchase so you will have to factor in its cost if you want the complete Galaxy Z Fold 4 experience.
