I shot the chart for those pixel peepers. I was shooting some photos side by side comparison of the D4 to the D5 at different ISOs.

I did a custom white balance using the ExpoDisc.

Click on the images to see their full resolution.

Nikon D5, Sigma 35mm ƒ/1.4 DG Art, ISO 100, ƒ/1.4, 1/30

Now ISO 400

Nikon D4, Sigma 35mm ƒ/1.4 DG Art, ISO 400, ƒ/1.4, 1/125

Nikon D5, Sigma 35mm ƒ/1.4 DG Art, ISO 400, ƒ/1.4, 1/125

Now ISO 1600

Nikon D4, Sigma 35mm ƒ/1.4 DG Art, ISO 1600, ƒ/1.4, 1/500

Nikon D5, Sigma 35mm ƒ/1.4 DG Art, ISO 1600, ƒ/1.4, 1/500

Now ISO 5600

Nikon D4, Sigma 35mm ƒ/1.4 DG Art, ISO 5600, ƒ/1.4, 1/2000

Nikon D5, Sigma 35mm ƒ/1.4 DG Art, ISO 5600, ƒ/1.4, 1/2000

ISO 12800

Nikon D4, Sigma 35mm ƒ/1.4 DG Art, ISO 12800, ƒ/1.4, 1/4000

Nikon D5, Sigma 35mm ƒ/1.4 DG Art, ISO 12800, ƒ/1.4, 1/4000

ISO 25600

Nikon D4, Sigma 35mm ƒ/1.4 DG Art, ISO 25600, ƒ/1.4, 1/8000

Nikon D5, Sigma 35mm ƒ/1.4 DG Art, ISO 25600, ƒ/1.4, 1/8000

ISO 51200

Nikon D4, Sigma 35mm ƒ/1.4 DG Art, ISO 51200, ƒ/5.6, 1/1000

Nikon D5, Sigma 35mm ƒ/1.4 DG Art, ISO 51200, ƒ/5.6, 1/1000

ISO 102400

Nikon D5, Sigma 35mm ƒ/1.4 DG Art, ISO 102400, ƒ/5.6, 1/2000

My opinion is that the D5 is giving you between 2 to 3 stops higher ISO with similar noise as the D4 at those lower ISO numbers. I would say that the D5 25600 is about the equivalent of the D4 ISO 6400. However, you can make that call yourself from the photos above.