It's not in the rulebook anywhere, but on the website Tech Bulletin TB-2018-001 says the 400 can run in LW Twins, 450 Superstock, 450 Superbike, and Formula 3. All the associated rules for each of those classes would apply.
I don't know what (if any) rules concerning the 400 will be altered for 2019 though.
Class fit for 400cc bikes has been hotly debated, and obviously the rulebook is not out yet.
That said, if I have understood correctly they will now have their own classes (400 SS and 400 SBK), and be eligible (AFAIK) for Formula III and Lightweight Twins.