yes, everything works fine... cant see any problems. first i wanted to revert, because the new gui is a bit confusing when you never have seen it. but now i think its better and more intuitive than the old one.
i have no idea why hik is not releasing the firmware for the k series... i had many hik cams and nvrs, and if i wanted to flash a wrong firmware which is not for the device, it simply does not work. so i assume that the k-series are in the header of the firmware. my device is only a few months old. not sure if this works for older devices.
if you want to test, i suggest to only flash if you have a TTL adapter. than you can revert if anything went wrong. just make a backup of your config and maybe swap a new harddrive in. i am not sure if you can use your old recordings with the new firmware.
(anything what you do is on your own risk, i am not responsible for any problems/defective hardware).
Hikvision have released V4.21.005 for their K-series NVRs (only 76 77 86 NI-K2 K4 K8) , which you can download from the UK portal here: HIKVISION UK PORTAL I have checked: Is this correct? V4 firmware for the K-series NVRs at the UK portal? If so, do you have any Release Notes for it...