Converting usually means resizing.
What NLE software are you using?
Where's the footage come from?
You would down-size the 4k resolution to 1080, rather than crop it.
RedCineX Pro does it for you, for RED originating files
Haha. we meet again, Well I am trying to understand the format if I am going to use the format after the discussion in http://www.indietalk.com/showthread.php?p=342805#post342805.. so yeah I know what 4K is, but I don't know a lot about 4K
4K is a beast to work with, and requires a hell of a lot more computing power than 1080, especially if you're looking into raw.
If you create a 1920x1080 sequence, then drag your 4K sequence into that - RIGHT CLICK and select "Scale to Fit Frame Size" and it will size it down to 1080P.
Either that or simply select a 1080 output module from ADOBE MEDIA ENCODER and it will size the 4K timeline into a 1080 one.