Re: Furnace filter
As previously mentioned, the 4" filter provides much less resistance to air flow than a 1" filter (because the surface area of the media is much larger). Newer, high-efficiency furnaces tend to have lower fan speeds, which means the air isn't sucked through as powerfully. A 1" filter may restrict the air flow too much for the furnace to operate a peak efficiency.
As for putting a "prefilter" at the return air vent, I think I'd use one of the mesh filters rather than a pleated one. It will trap the largest dust without restricting the airflow as much as a pleated filter would.
Any filter, no matter how coarse, will still trap some amount of fine dust because air moving across a surface creates a polarized static charge in the surface (in this case the filter media) and an opposite polarized static charge in the dust in the air. This causes the dust in the air to be attracted and stick to the filter media.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.