I just got off the phone with Microsoft support and the issue of intermittent pen not writing is a "known issue" according to MS. My pen only works reliably after a fresh restart of the tablet, otherwise is less than 50/50 reliable. They said they are working on it and will keep me posted. I will be returning my device within the 30 days window if no fix in the next couple weeks.
I can still bring up OneNote by clicking the eraser, even when the pen won't write.
I hope they fix this because I want to keep this SP4 otherwise, cost me two grand
Exact same problem, pen tip stops registering touch or drag. I bought mine in a physical store so I got them to swap just the pen. While I was there I noticed that they had the blue one in stock, so I bought that also. At first the screen would also stop showing the little hover dot. Now it shows the dot but does nothing when you touch the screen (some of the time).
Same problem has recurred with the replacement silver pen as well as the new blue pen. So either it's in software, the screen, or all three pens are bad.
Now here's the kicker: I still have a spare pen from the SP3 that I got rid of. (Purple tip.) This pen works great, never stops working. I recorded a small video I put upon YouTube showing the silver pen doing nothing at the same time as the purple tip works fine.
My SP4 pens generally work again if I unlock the pen top and just draw it back enough to disconnect the battery it works again. Also lightly tapping it on the side of the tablet sometimes gets it working again. I'm not confident that these fixes are actual rather than coincidence, but one thing that's absolute is that the SP3 pen has never taken a break, only all three SP4 pens.