Is it possible to upgrade ionic from Version 1.X to 4
Yes and no.
No, there is no way to run a command or follow some simple steps. Ionic v1 is AngularJS, Ionic 2,3 and 4 use Angular - which is a totally different thing although the similar name.
You can use parts of Ionic 4 in Ionic v1 project though, as @ionic/core are just web components. Googling should give you some examples and tutorials.
So we need to rewrite all the modules ?
For an upgrade, yes.
If you “just” try to use some of the Ionic 4 UI components via @ionic/core, probably not.