Romain is a good source if he says it's not in development yet then it likely isn't. Also consider they've got the DLC to work on. Its clear Shenmue IV had been talked about but there's a ways to go.
Personally I think it will be a slow burner in terms of sales. We've seen that its seemingly selling well in the USA due to the price cuts and the limited editions flew out of the window.
Secondly we're all likely the early adopters to the game. We know about it, following news etc, theres plenty of people out there still discovering the first 2 games and they can then move to the third quite easily. There's more sales.
Thirdly, as a community we need to keep pushing this game. Yu Suzuki is right when he says try and get 1 person to buy Shenmue. Theres 80k kickstarter backers for one, why can't those people get a friend involved and generate more sales? As a community, and I'll keep repeating this, we dont stop now. On Dec 3rd Shenmue day we go hard, push the game, the good reviews, let people know about it. We didn't get Shenmue III by staying passive.
There's also plenty of people who bought the game but didn't back it, let's get them involved. Twitch streamers, etc let's get the word out there.
For the record I do think we will hear something in the coming months about sales and a potential for a Shenmue IV but we have to hang tough a little longer.