Is it possible to edit existing forms in V4 form builder? - YES

Hello Robert, 

Yes, you can open existing forms in V4 builder. To do that please follow:

1. Go to Settings of your account:

https://www.jotform.com/myaccount/settings

2. And simply change Form Builder to V4:

After selecting that option, your form should open in V4 builder.

If you have any further questions please let us know.

Thank you!