4K video or 3840x2160 is only equivalent to 8mp of actual resolution. The 7 Plus takes pictures at 4000*3000 resolution.
You would have to define under what criteria you deem the video to be better than the photos, from the limited information you provided I'm willing to bet it is due to the processing/noise reduction applied to still images.
In that case, I would recommend shooting in RAW with an app like LightRoom and adjusting the image to your liking in post processing assuming you are familiar with photography and RAW image manipulation.