Product review (5 stars): Fits Aprilaire 400 Humidifier perfectly
This replacement filter fits the Aprilaire 400 humidifier and is cheaper (including shipping) than purchasing from aprilaire's website (even though they have free shipping).

That being said, this replacement filter is grossly overpriced for what it is.  Clearly a profit center for Aprilaire.

I have ordered this part before and have had no issues with this seller.  Product arrived quickly.