Probably a 1". I don’t recall seeing a 4". I just checked Target.com, and they only have 1" filters in stores. Where did you get the 4"?

I tried that single-filter design in the past, and it seemed to do a poor job of filtering the air, and it stressed out the fan motor a lot.

I tried the single-filter design some years ago, and it worked pretty poorly. It didn’t seem to do a good job of cleaning the air, and it stressed out the motor a lot. I’d consider this 4-filter design for the future.

I was just thinking about creating a spreadsheet with people’s contact info, notes, and a picture. I guess an app would be preferable, especially if there were a web/desktop version, and it was compatible with a spreadsheet (can export from/to a spreadsheet).