I find it that in natural light, the ISO can be increased higher than in artificial (low K, possibly discontinuous spectrum ) light before the image falls apart in terms of tonal and colour quality. I suspect this is because silicon photodiodes are less sensitive to blue than red and near IR light. Thus if the light is reddish, AWB increases the blue channel values to get a more neutral result, but in doing so it increases the noise of the channel that is weakest and so there is objectionable noise. In bluish light (skylight) the red channel is amplified but this is not as much of an issue as it has high sensitivity and SNR.
With the D5, I have gotten good results at up to ISO 25600 (D810: 6400) in waning daylight. However at the said ISO settings, in indoor artificial light, I've found the results "thin" and of poor quality. So I would say I prefer to stay at lower settings for indoor available light photography, or if I must, I will turn the results into black and white which reduces my objections to the noise a bit, also then there are no more colour quality issues. However, the new anti flicker mode in the D5/D500 may change this and allow shooting in flickering artificial lights when previously the results were poor or highly variable.
Sorry I have no real experience with the D500 or D4.