block off the one under the unit by half so it starts to pull that 15" run back more..is the furnace in a louvered door room or 100% open that 15 run seems more the true house return.if you want to go with 4" filter to catch the dust it can be done you can go with a 2" pleated 1" pleated also check out local Grainger supply they welcome home owners...the unit is basically pulling the most air from the nearest source as it should and the filter differece is the proff:whistling2: