The first time I ran iwlist, I got results. Every other time: What's that about? And where is it even looking so I can look there myself and sort this out?

dmesg is usually a good start to investigate

Yes it usually is.... all 1,053 lines of it. But I didn't find anything out of the ordinary.

I was thinking iwlist was reading some file 11 directories deep in /proc or /sys. The only thing I found was /proc/net/wireless, which is very basic.

Sometimes "iwlist" is not compatible with some hardware

maybe you can try

Thanks for your ideas. Sorry to be shooting them down, but the 'iw dev wlan4 scan'
returns a list of device capabilities like iw list. As for the hardware That's exactly the same wifi chip as lives in my elderly laptop (also on Slackware-15.0) and the laptop works just fine, and does a channel scan. The working one is an onboard chip, the not-scanning one is pcie card.

As the error shows "Failed to read scan data," I tried 'iwlist wlan4 scan >iwlist.txt.' But that gave me a null file.

Being basically a 'fixer' of some form all my working life, the gremlins that want to cause me trouble have to rise to the challenge. They do, unfortunately, so I rarely post simple problems.

It's always a good idea to consider all possible solutions

The matrix :
simple problem => simple solution
complex problem => complex solution

is rarely true

You don't know how many times the simplest solution has solved my problem, regardless of complexity ;-)

And the simple solution to this one is...?. Don't worry - I didn't have it either!

The wifi must cycle through every available 2.4G & 5G channel (Other frequency bands and channels are disabled) and get details from every SSID within range. It's returning no information, (= "Unable to read.....") but my old laptop on the same chip, driver & kernel reads with no problem.

So I tried with a Liveslak usb key holding Slackware64-current. That was unable to read the scan data from my box. Then, being thorough, I plugged Liveslak into my elderly laptop, and it listed the stations.

So it's a hardware issue, perhaps my pcie card not seated right or some other issue. It's certainly not the first thing I would have thought of. I was condemning myself as a 'Sorcerer's Apprentice' type of sysadmin, which is the usual story.

man rfkill

That tells me I'm not hardware or software blocked, but still no list

Maybe try while scanning:

and
You may be to close to AP, or have some sort interference with ethernet card (assuming that you have one)
Ath drivers, if this is something new, check out older version that worked before or whatever newest/experimental.

That box is in for a compulsory pit stop. We'll have to pause this, and see what comes out the other end. Thanks for your thoughts.