About a few hours ago, Walmart accidentally posted many, many games on it's game page on it's website that aren't released yet, most likely by accident, like you can see with the image posted. If you look at the 2nd row, 1st column you see Just Cause 4 on there. Some of these games like Division 2 and all seem to already be confirmed so if Just Cause 4 is in these pile of already or possibly confirmed games, surely Just Cause 4 has been in the works for quite some time now.
Anyways, just wanted to see what you think on the issue, but from what it look to me, Just Cause 4 might be coming earlier than the 2019/2020 expected release date, or at least it's looking even more probable that it'll happen in the next year or two.