Why don’t you want to do this as a 5th year in HS - what are the obstacles you’re thinking of? This opportunity can change the trajectory of what you want to do in college, so if you think you can handle this experience at your age, this could be really helpful to do this now.
I also want to mention that it’s really HARD to live in a country with a different culture & language, even though it’s so interesting and even though Japan is safe and a 1st world country. So the idea of spending a month away from the US in the summer won’t really help so much with your college application, but could be invaluable in helping you decide whether you want to take this big of a time commitment away from the US while you are in HS.