Predict based on what you think Japanese sales will be for 2019 based on Media Create for the Switch and PlayStation 4. Everything else is dead, or near death, so that is why I left it for the only two alive.
Switch:
2017: 3,312,619
2018: 3,534,165
PS4:
2017: 1,791,736
2018: 1,726,031
My own prediction (based on various things):
Switch: 3,753,216
PS4: 1,381,566
First week of Japan so far:
Switch: 199,982
PS4: 117,182
Bonus: PS5 launch 2019 in Japan? Nope!