It's restricted not for emissions but for noise regulations due to all the "loud pipes" in U.S. Bikes meeting stricter Euro-5 emissions standards gets full power. Harleys are whisper-quiet in Europe. We get de-tuned version ZX-4R with 56bhp instead of 79bhp available elsewhere.
https://www.motorcycle.com/new-model...-4rr-make.html
https://www.cycleworld.com/motorcycl...g-to-usa-2023/
I saw one couple weeks ago on ride to South Mountain.
He's going to be going to trackday at AMP on 10/1. Will be interesting to see how it compares to Ninja 400's time of 1:38.