There are dozens of different paths and strategies to gaining a readership. Listing all of them would take some serious doing, but I can give you some easy examples.
Get published by a major house. The path here is: Finish an entire novel; polish it as much as you can on your own and with beta-reader feedback; find an agent; find a publisher. Being published gets you readers because your book is in bookstores, because you've got the publisher's marketing, because you start tapping into other publicity-generators, like conventions or interviews.
Self-publish frequently and inexpensively. One route to self-publishing success is to write a series of fun, entertaining pieces, which are relatively inexpensive (you control the price!) and might each be fairly short. In this way, you build up a backlist of books and stories; readers find you when they're bargain-hunting in your area or genre, and are drawn in by the low price (and, hopefully, some good initial reviews). Then they want to keep reading, because the books are fun and the price is low!
Invest in social media. A lot of authors build up their "brand" on social media -- by doing stuff on blogs, Twitter, podcasts. People come across them on their everyday internet browsing; and the ones who key onto that particular author will (hopefully) go on and check out their books. This takes a lot of work and attention that isn't directly on your fiction-writing, but it's doing your own marketing, and reaching out to potential readers basically wherever they are. Note that this kind of activity isn't just saying "read my book; buy my stuff" -- because almost no random internet browser will be interested in an author who has nothing to say but "read my book."
In all these cases, you're figuring out what kind of audience you're marketing to -- bookstore-visitors; review-readers; Amazon-browsers; social-media-followers -- and figure out what's required to get their attention.
Here's the major thing for your particular case, though: Right now, you're not offering a lot. I'm sorry if that's hard to hear, but: right now, you have a single 4-year WIP. That's not a lot to appeal to readers, whatever your audience -- the readership for stories that aren't finished yet is vanishingly small. Reaching them (and standing out from another million WIPs on Wattpad) is extremely difficult.
This is not any sort of insult to your writing -- quite the opposite. Your writing can be fantastic, and you still won't have an audience for an online WIP. It's worth thinking about what kind of audience you'd like to draw, and why you even want a lot of readers when your book isn't even finished yet -- that might point you in new directions.
Hope this helps, and all the best :)