This price increase is live for new subscribers as seen on youtube.com/premium. Instead of $11.99, YouTube Premium now costs $13.99/month. Meanwhile, it’s $18.99 if you’re subscribing from the iOS YouTube app.
I’ve been paying $4/month over market rate this whole time?
Yeah. That's normal if you subscribe to things through (iOS or android) apps.
Google and apple don't allow apps you roll their own payment methods to "protect their users". Apps have to use the Google/Apple payment system where Google/Apple take a 30% cut of every payment.
With YouTube being owner by Google they probably don't have to pay those 30% on Android, but they have to on iOS.