KPXG did have its ATSC mux on channel 4 (as 04-03 to -06, PSIP'd as 22-01 to -04) until last October, if I remember right. They then shut its NTSC broadcast on 22 and moved its ATSC there, where it remains today.
Currently there are no more low-VHF ATSC's in Portland. I doubt there will be again as far as I can see.