The filter that is currently in my furnace is 16x25x5, and I was wondering if it would be ok to use a 16x25x4 instead.

I am having a hard time finding a 16x25x5 locally. Lowes and Home Depot sell the 4" filters all day long... which would make it much more convenient to buy a filter when I need it.

Or.. for future filter needs, if anyone would be kind enough to provide a link to a reputable online store... please do.