I expect Japan sales of PS4 at launch to be slow ( high sales, but drops to low numbers subsequently ), but at the same time, I expect it to have long legs and will not bomb.
In terms of games that will most likely migrate entirely to next-gen earliest ( western devs ), even though they don't do great numbers, top-tier western games do sell reasonably well in Japan. Nowhere near the best-sellers, but there appears to be a healthy market of gamers in Japan who likes their CoD, Battlefield, GTA, GoW, TLoU, etc.
Cumulatively, I would expect that audience alone to be at least 1mil+ in Japan.
Locally, we're seeing that the most popular console franchises ( with global appeal ) are still sticking to their console roots. MGS will be on consoles for the foreseeable future. So will FF, KH, Biohazard. These titles, probably 2mil+ audience.
Even games with a very local flavour like Yakuza are still sticking to consoles for the time being. From what we know and hear, games like Tales mothership is staying on consoles, etc. Who knows if things change for other franchises in the future, but there's still a good amount of franchises that sell well that are staying on consoles. Said games are also small enough that they don't need 1million sales to be successful.
And then there's all the niche franchises. EDF, IDOLM@STER, Dream Club. Many of these games have no issues coming to PS4, and be on all Sony platforms due to their limited fanbase, low sales expectations, and the fact that multiplatform approach is quite successful in Japan, with Vita + PS3 sales both being healthy on their own right despite games appearing on both platforms.
My gut feel for Japan is somewhere along these lines:
Does well@launch, but won't be near PS2 launch sales.
Slow sales for 2014.
Sales pick up in 2015 as more western games stop being cross-gen, FFXV/MGSV has clearer dates, and GT7 is announced.