Adapted my 20x20x1 air filter return to a 18x18x4. The only problem was the old grate restricted air and it caused a whistle. I replaced the grate with a newer one and the whistle went away. The whistle was because more air was moving through the new 4 inch filter than the 1 inch filter. Better for the furnace motor.
I purchased these as replacements for the Filtrete filters I currently use. The pleats on the Filterbuy filters are not evenly spaced and there are fewer of them (10 verses 14) so there is less material filtering the air. As you can see, the Filtrete has spacers that keep the plates aligned. I thought that I was purchasing a direct replacement.
Can't just give it five stars; they need me to say something, so I've bumped it down to 4 stars. I'm supposed to get really excited about these filters;they arrived, they were expensive, they fit and they last about 2 months in the winter before they clog up, They ARE less expensive than other brands,the end.