Just got a new Trane Package Heat Pump. I had read a discussion of air filters here a few weeks ago, and requested a 4 inch media filter. The contractor at first said I needed a bigger filter grill because my present one is 20 x 25. Once he saw that he'd have to cut into wall joists his story changed to me not needing a bigger size. Manual says 4 sq feet with a w.c. between .05 and .08 (it was contradictory) at a 300 fpm flow rate. I checked and the honeywell 4 inch Merv 10 filter has a w.c. of .21. So I'm in a pickle. Even with a bigger grill (which I'll have to demand), I'm way over the recommended w.c. rating for my unit. I don't see how any efficient filter is going to meet spec. Any advice?