1) For low light shooting Sony is claiming the a7S II is better by approximately 2 stops than the a7R II.
2) The only difference between the sensors A7s and A7sII sensors is that the colour science has been tweaked on the a7S II and should offer results that are similar to that of the a7R II. This is something that a lot of Sony fans will be very pleased to hear as the a7R II seems to offer more pleasing skin tones than that of the original a7S.
3) A7sII has “S-Log3 Cine” with more filmic look. S-Log3 or S-Log3 Cine gammas has been lowered to 1600 ISO instead of the 3200 for S-Log2 on the a7S.
4) Strangely enough there is no option to record 4K internally in APS-C crop mode, you can only record 4K internally in full frame mode.
Update: obviously in crop mode there are less pixels so 4k is impossible :)