[Kiwi Rider]

The new Ninja 400 is going to be allowed to race in the World Supersport 300 class of the World Superbike championship next year. There will probably be a minimum weight penalty and maximum rev limitation on the 400 to keep the playing field level. The Ninja will be the second most powerful bike in class next to the Honda CBR500R which currently has it's revs limited to 9,500RPM.

Kawasaki Ninja 400 to Compete in 2018 World Supersport 300 Class

 

[Chappy]

It's just a tad over 300cc, so I'm sure the Ninja 400 will be fine. The article did say this: "Left unsaid, however, is the irony of the WSS300 class no longer having any bikes displacing below 300cc." So all it may need to do is meet the weight requirement.

 

[xorbe]

Yamaha needs to respond with a 321cc R3 that revs to 14.5 and makes 48 hp. ;-)

 

[Kiwi Rider]

Yamaha needs to respond with a 321cc R3 that revs to 14.5 and makes 48 hp. ;-)

That would be good to see but it's more likely be a 399cc R4 that revs to 12.5K and makes 46HP .
I could be wrong, time will tell.