I'm pretty sure the F400 *can* do 4G in North America. I have the 400S and 4G (LTE) works in Canada, and Canada is in North America (I think). And, I read posts somewhere of people on AT&T getting 4G.
So it's wrong to say that 4G does not work on the Korean models of the LG G3 in North America.
Yup. All the F400 (S, K, L) models will run on AT&T's LTE Band 17 (700 MHz spectrum). As a lot of the resellers of the phone are not familiar with usage of the device outside of Korea, they provide the "This phone's 4G may be incompatible with your country's 4G" statement as a means of insuring their sales. A lot of them are unaware of the extra band support found in the korean variants...
I've had access to my F400S, and another F400K (in the recent days), and can tell you that they have no issues connecting to AT&T's LTE. This however does not mean it is 100% functional with AT&T's LTE, as they offer LTE on multiple bands, and not just Band 17. You will need to do some research on your locale
http://www.wirelessadvisor.com/. Look at which AT&T LTE bands are supported, and you can hop on over to the other Q&A thread I started a bit ago with info on all available bands on the korean variants
http://xdaforums.com/showthread.php?t=2784967... Check out post #4 (Not trying to bump my own thread, but too lazy to type out all the bands again... lol
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