Is 4 years even possible? I’m nearly 15 years in and probably know about 1500 kanji (not using WaniKani). I feel like all these stories really sensastionalize how much people really know.
WaniKani works because of the spaced repetition. I think it's easier to learn through level 60 in 2 years, rather than 4+ years because it's paced for that, and that helps you actually memorize them.
Note that I also went to a language school full time for a year, which helped with vocabulary, which obviously makes memorizing the vocab kanji easier. I also moved to Japan when I started learning, so I'm forced to read Japanese all the time.
Also worth noting that I do 200-300 reviews per day. If I'm on-top of my reviews, it's possible to do 300 reviews in about an hour and a half. When I stopped for a couple months, it would take me 3+ hours to do 300 reviews, because my correct percentage is 20-30% rather than being 85-95%. That alone shows me that keeping a proper review cycle helps me memorize.
Although I'd say there's not really a point in saying one 'knows' kanji, the bulk of your understanding comes from knowing words. There's so many of them reading is going to be a struggle anyway, but it slowly gets easier.
Obviously that includes grammar and other things too, but grammar isn't that big of a deal when the sheer volume of characters required to know and contextualize takes care of most.