So, Wanikani has its own system. The reasons for that system have been covered above, but mainly you start at the start because everything about the system of learning kanji/vocab at Wanikani starts from there. In certain ways, Wanikani is teaching its own version of Japanese as a bridge to get you to learning real Japanese. For example, finding Charlie Sheen’s heart in a rice field and eating it, thus dumbing you down to pre-Einstein-heart levels of thought isn’t exactly standard Japanese. It may seem silly, but if you jumped into the middle of this craziness, it wouldn’t make any sense. If you start at the start, though, it almost does!
But anyway, what you need to ask yourself I think is whether the cost/benefits of the system would work for you. If you managed to get to N3 level of kanji/vocab knowledge within a year using some other method, then that sounds like a pretty good method, and maybe you should just stick with it and keep learning that way. But if it took a few years to get to N3 level, then Wanikani might be worthwhile as a way of building more kanji/vocab more quickly and efficiently.
I can’t speak for you, and I feel like N3 is quite a bit higher than N4, so take my experience with a grain of salt. But! I think (but haven’t proven) that I could pass N4 reasonably easily, and felt that way before starting Wanikani at the beginning of this year. In less than 1.5 months, I’ve made it to level 6. A lot of what’s been covered have been things I already knew, but I’ve also learned a whole lot of new things, and I haven’t been bored at all. Also, many things I sort-of knew have become solidified in my brain in a way they never were before.
You speak of going to Japan in September as though that’s some kind of cut-off time for learning Japanese, but of course it won’t be. You’ll learn a lot more once you’re there, and you could keep using Wanikani there, and it will prove even more useful there as you’ll encounter words you learn “in the wild” much more frequently there.
My guess is that even though a lot of the early levels are review, the system may well prove useful and maintain your interest. But the only way to find out is go through the first three free levels and see how you like it. I think at level one, it’s too early to tell, and it certainly does get more challenging as you proceed. Good luck.