The just released Sony FDR-AX100 Handycam is not just a 4k recording multipurpose camera but it also hides a 720p 120f mode which so far seems to be lower quality than regular 720p video cameras. The quality while usable is not ideal. The resolution seems to be well below the stated 720p in slow motion mode.
There is a sister model with the same sensor called the HDR-CX900 which shares all features “Including 720p 120F recording” except the 4k recording and of course a discounted price. However the $500 shave might be unwarranted considering the 1.5k investment for the CX900 which for $500 more you get 4k on the AX100.