actually, there is no difference in terms of sensor type btw the 4K and the 4k+: both use the Sony IMX377, which produces an image which is 4032 x 3024px in size (12.2 MP), with a lens having a FOV of 155°.
you need a 3840x2160px image, and this is what you get when using wide setting; when going for medium fov, you crop the more central part of the sensor, which ultimately produce an image which is "slightly" with less horizontal resolution, thus not real 4K (i.e not real 3840x2160p).
I just wonder why, in the new Xiaomi Yi 4k+ at least, Xiaomi did not decide to go for a different Sony sensor, considering there are better sensor with a 16Mpx resolution which could have fit the purpose.....let us hope they soon release a new version of the 4K+ with maybse sensors such as IMX400 or IMX398....