As you may know, IS-4 will be moved to tier 10, side by side with IS-7, but it will have a new gun.
The new gun is the 122mm M62-T2. Here are the official gun stats, provided by a WoT dev.
A more direct comparison between S-70 and M62-T2:
UPDATE: Looks like these specs are not actually final. Overlord had this to say:
The specs do look scary indeed and they are not final yet and will undergo some tweaking prior to release.
IS-4 in it's full glory as tier 10 heavy: