You need to follow below steps to migrating an existing application from Ionic 3 to 4.Steps are follows -
- First, Install the latest version of Ionic
$ npm install -g ionic
The -g means it is a global install.
- Now create a new project using the blank template -
$ ionic start myApp blank --type=angular
Please check full example on this URL:https://allexperts16.blogspot.com/2018/08/migrating-of-an-existing-application-from-Ionic-3-to-4.html