83% AFUE 95,000 BTU Output Front Flue Oil Highboy Hot Air Furnace

The filter housing is not part of the furnace. Granted I do know that there are and were some furnaces in the past, that did have internal filters but that is typically not the way it is done. Instead the filters are sized to the system and each system is different. Today the HVAC contractor installs a "filter rack" between the return air cabinet and the furnace. So lets assume this is a new installation. The contractor would size both the plenum and the duct work leaving the furnace that brings the warm air to the different rooms. This is sometimes called a duct reduction because after each branch the duct gets smaller in order to maintain the velocity so that the hot air reaches the furthest rooms. In a similar fashion the return air duct work, filter size and filter MERV rating would be sized and would depend on the size and length of the duct work. By sizing it properly you insure the proper speed and volume of air. The is not rock science but there is math to it. If not sized properly the air moving through the duct can make and excess amount of noise. Conversely if the unit is starved for air the furnace could over heat. My point is this same furnace could be used in ten different installations and each could call for a different filter size and thickness.

Please refer to the specification sheet found under the header "info and guides" on the Home Depot product listing page. If you open the PFD you will find it contains a legend to decode these numbers. However the answer to your question is that is a revision code. So the last time they improved the unit it went from the "3A" to a "4A"