I have (and maintain) an Ionic 3 app, and want to build a new app with Ionic 4. Can I run both Ionic 3 & 4 CLI on the same system?
CLI versions are completely independent of framework versions. At least according to my understanding of the official Ionic team position on this issue, any version of the CLI newer than the one used to create a project can be used to build and maintain that project (and therefore, if there are problems, that constitutes a bug in the CLI).
So, while the answer to the actual question you asked is “yes”, I think what you really want to know is “what do I have to do in order to work with both framework 3 and framework 4 apps in the same development environment?”, to which I would say “nothing extra”.
Thanks for the explanation. Makes sense.