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.
It's really not sensationalizing. I'm level 50 on WaniKani at 2 years in, and had a couple 2-3 month lapses (with 2000+ review queues - I'm currently working one of those off). If I wouldn't have had the lapses I'd have finished a few months ago at my normal pace.
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.
Sure? I did WK in 1.5 years and then just started reading stuff. Now I'm about 4 years into studying and read about 30-40 books. Wanikani is just the tip of the iceberg, as it doesn't give you nearly enough vocabulary to be comfortable with texts. Of course I did spend a lot of time listening, too, which helped. I'm just now starting to not have to really lookup stuff when reading Murakami or Higashino Keigo. Still see new words every day, though.
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.
The more extreme stories claim to pass JLPT1 in 6-8 months investing 6 hours or so average a day. That's about what, 1-2 new kanji an hour at about 1k-1.3k hour investment? That's 1 hour a day consistently over 4 years.
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.
I think with WK getting to 1.5k - 2k is possible in 1-2 years if, as OP said, you invest a lot of time every day. I noticed that if I could keep my usual pace every day I would complete it in ~2years. But since I do take breaks, have better and worse days, I am at a 4-5 year pace. I am at level 32 and about 2.5 years in.