You might remember Murazor has mentioned a big change for the IS-4 branch in a recent Q&A. He has now confirmed his plans to change the branch and move the IS-4 back to Tier IX.
Not a lot of details have been shared, but currently developers are talking about changing the entire branch and the possibility of introducing ST-II into the game.
Please remember this information is unconfirmed and not definitive that will make it to the live server.
Source: WOT Leaks VK