In Ionic v4 ionViewWillLeave still exists
For reference: https://github.com/ionic-team/ionic/blob/37c9be7453ceb0e34b28f9c7338c77b90b626d19/core/src/index.ts
I think the confusion comes from the documentation which could be a bit misleading, according the amount of question about this subject. The only lifecycle, as far as I know, which doesn’t exist in v4 anymore is ionViewDidLoad, this has to be replaced with ngOnInit