Furnace filter ?
The 4 and 5" media filters are pleated. With that many pleats. it has a much greater surface area for the air to go through.
The surface area is a single pass.
If you used 4-1" pleated air filters. The surface area is in series.
In series, this becomes a greater pressure drop, and restricts air flow from day one.
An OEM 4" media air filter may have a pressure drop of .15" brand new at 1200CFM.
4-1" pleated air filters may have a pressure drop of .3" or more brand new at 1200CFM.
While the 4" may load up in 6, 9, or even 12 months. The 4-1" filters will load up in 20 to 30 days.
So you would end up replacing them atleast once a month. So multiply their cost by 12, is it realy any cheaper.
Not to mention the harm it could do to your equipment by reducing air flow even when new. Let alone 15 days later.