The Sony a7S III or a7S IV is expected to be announced in early 2020 as already rumored before. Now according to SAR, first leaked image of Sony a7SIII is already available online. You can see leaked image above.
According to rumors, Sony a7S III will featured with 4K 120p video, but no 8K video. There will be also a cooling system in a7S III as Panasonic Lumix S1H. Stay tuned for more info.