WaniKani is a paid service after level 3.
If you decide to purchase a WaniKani subscription, you get access to a lot of (cool) stuff!
Premium WaniKani content currently includes levels 4-60. You also get access to Vacation Mode, which lets you pause your reviews if you’re going to be away for more than a day or two.
What subscribing doesn’t do is make WaniKani go any faster; you’ll still need to do your reviews and level up like everyone else.