Letterbox when converting 4k to 1080p Got handed a pile of stock footage downloaded from various sites. And found one clip is 4k ( 4096x2160)

I put it into AME to reexport it as DNx115 but when I set it to 1920x1080 23.976 I see letterboxing top and bottom. It says the source clip pixel ratio is 1 and mine is 1.. tried the others for kicks but none get rid of it. 

Why am I getting letterbox for this conversion?
Because 4096 / 1920 = 2.13333 and 2160/1080 = 2.  They are different aspect ratios, so yes, they end up letter boxing when you convert between them.  That is the expected result.

The 4K format you are using is a multiple of 2K and is not the same widescreen aspect ratio as 1080p.  You can't translate between them without letterboxing, though there are several different competing "4k" resolutions, so it won't hold true for all "4k" formats, but it does for the one you are working with.