I bet it'll take at least 2019 to release on PS4, especially if the game stays in early access till 2018, and with the low staff count scrambling to finish the PC version and essentially port to Xbox.
Even if it's 1 year exclusive, I bet the timer doesn't kick in until the game is out of early access. Start counting for PS5 port (or PS4 port forward-compatible with PS5).
Sony isn't going to keep this game off the platform because it carries an 'early access' label. Come on now.
I bet both console versions of the game end up hitting in 2018 with a 6 month delay between them or so.