Filter Sizes

I'm baffled by the range of choices for furnace filters.

A new furnace that I'm having installed will have a custom built 20x20x4" media cabinet. But looking around, I'm seeing that the 4" filters are actually undercut to around 3 3/4"...which I assume will fit in my cabinet. To make matters even more confusing, there are some "so called" 4" filters that are really 4 3/16" to 4 3/8" that evidently are for the Honeywell cabinets. Not sure though. Where I'm really confused is that the 4" undercut filters seem to be pretty generic and not the best quality, again I think. Is the 3 3/4" size even standard so you can pick one up at a local store ?

Merv Ratings

I'd like a better Merv rated 4" filter....say around a Merv 10-13, but I'm worried about 2 things:

1. Air resistance. Will it even work, or will it slowly destroy the furnace ?
2. Is it realistic to change a Merv 10-13 filter every 6 months ?

If not, then perhaps I should stick with the cheaper Merv 8's and change them more often ?