Well, here it is guys. The 4k vid recording will only be available for a particular mode a la capture mode and only with chipsets supporting SD800(Lumia 1520/930/Icon).
830 will only have 1080p moment capture mode and the stills you extract from it would be of 2mp.
http://conversations.nokia.com/2014/09/05/lumia-camera-focus-capture-moment-quality-ease-speed/
http://img.tapatalk.com/d/14/09/07/tyta5yhy.png
Edit: Sorry. It has been posted before. Didn't read the whole thread.
Protip: Do read the thread a little . Thanks anyway..