For the record, I know what 4:1 compression sounds like. I dubbed thousands of those files for hundreds of format libraries, over the 10+ years the company was sending out 4:1 MP2s for the over 1200 stations that were clients of the company I worked for.
4:1 compression removes some musical information from the stereo spectrum. You usually hear it only on headphones -- the lost or reduced cymbal hit, the missing percussion in the background, the sounds that file compression reduces to decrease the file size.
Then you hear the next song in full stereo, in high resolution, with all the instrumentation clearly audible, just like the CD. Which is what I heard on KPNW -- a few tracks here or there were clearly missing information, sounding like a 4:1 MP2. The rest of the tracks, audio processing notwithstanding, sounded just like the CD.
Same audio processing. Different resolution in the sound files.
If what I heard was the audio processing, it would have been the same on every single track. It wasn't. Some of the tracks sound like 4:1.
I only heard it on three or four tracks over a period of two days listening (a couple hours at a time) -- all with headphones, so if KPNW is using older, compressed files on some of their alt library tracks, it's not many of them. I also heard full mono tracks, when there are stereo versions available of those songs online. The point being that as KPNW's library is varied, they may have some older sound files included, in older formats. I'm guessing if that's the case, those files are probably being replaced (or removed) as the station continues in this format.