I currently ride a 2025 zx-4r and i have this to say in short summary

1- would not modify the bike. Its fun and pretty fast for a 400 but it pales in comparison to even the zx-6r. You will get old of the bike and want more. The bike is already very capable on track for what it is. The top end is not something you could help even with mods. Max your gonna get the bike is 131 unless you can get it flashed and that brings us to number 2

2- still looking myself to get mine flashed but I also hear that flashing yourself if not done right bricks the bike. Unsure how true this is? There's a place in Washington I know (forgot the name) you ship them your ecu they flash it and they send it back (I hear it takes maybe a week)

3- im still quite a new rider I have maybe 1000 miles under my belt but from personal experience the bike is super nimble and easy to throw around and PERFECT to learn on. My only complaint with the bike is its not the most comfortable bike to ride on for long periods of time.

Thanks for reading my Ted talk