The new version 4.0 firmware for the PXW-FS5 brings a new lower base ISO range to the camera. This very slightly reduces noise levels in the pictures. If you use “gain” in dB to indicate your gain level, then you shouldn’t have any problems, +6dB is still +6dB and will be twice as noisy as 0dB. However if you use ISO to indicate your gain level then be aware that as the base sensitivity is now lower, if you use the same ISO with version 4 as you did with version 3 you will be adding more gain than before.

Version 3 ISO  in black, version 4 ISO in Blue

Standard 1000 ISO – 800 ISO
Still 800 ISO- 640 ISO
Cinegamma 1  800 ISO – 640 ISO
Cinegamma 2  640 ISO – 500 ISO
Cinegamma 3  1000 ISO – 800 ISO
Cinegamma 4  1000 ISO – 800 ISO
ITU709 1000 ISO – 800 ISO
ITU709(800) 3200 ISO – 2000 ISO
S-Log2 3200 ISO – 3200/2000 ISO
S-Log3 3200 ISO- 3200/2000 ISO

At 0dB or the base ISO these small changes (a little under 3dB) won’t make much difference because the noise levels are pretty low in either case. But at higher gain levels the difference is more noticeable.

For example if you  often used Cinegamma 1 at 3200 ISO with Version 3 you would be adding 12dB gain and the pictures would be approx 4x noisier than the base ISO.

With Version 4, 3200 ISO with Cinegamma 1 is an extra 15dB gain and you will have pictures approx 6 time noisier than the base ISO.

Having said that, because 0dB in version 4 is now a little less noisy than in version 3, 3200 ISO in V3 looks quite similar to 3200 ISO in version 4 even though you are adding a bit more gain.