I wonder is it possible to use Ionic CLI v4 to generate Ionic v3 template? or the best way is to downgrade CLI to v3?
Yes, it is possible with:
ionic start myApp blank --type=ionic-angular
I suggest you always use the newest version of the ionic CLI.
Yes, it is possible to create an ionic v3 template by using ionic CLI v4.
ionic start projectname --type = ionic-angular should do the job.