Thanks for replying. I did what you said and installed HWINFO64 and ran it for a while. And yeah, I was incorrect about the speeds. So forgive me, the cores where not idling around 4200, but more like 3500 MHz which is base clock speed according to the spec sheet at AMD.com. But, alternately, on my rig with an Intel 9980 xe, the base clock is 3000 MHz, and yet at idle it is cruising at about 1200 MHz. So what is going on? Do computers typically idle at base clock or less than half that? Or do AMD chips just idle higher?

I guess you just need to change the powerplan under Windows power options. Looks like you are running Windows High Performance powerplan. Install AMD chipset drivers from AMD and run the AMD Ryzen Balanced powerplan instead if you want lower idle frequencies.
I use the AMD High Performance powerplan, yes it doesn't go down that much in frequency but performance is mint. The frequency doesn't affect temps on idle anyway, only under load.