V4.5 runs on iOS 9.3 through iOS 10. We presume it will run on iOS 11 because nobody has reported any issues, but we are currently only testing V5 on that. Certainly, V4.5 is not optimized for iOS 11 per se.
V4.5 will always run on those platforms as it is the last of that line. V5 as we noted previously (see Announcements forum) is a separate app so it wont override whatever device you want to use V4.5 via automatic update, etc.
V5 will require at least 10.3 and at this point likely would require iOS 11. This is a list of iOS 11 compatible devices:
macrumors.com/2017/06/06/io … od-models/
V4.5 and V5 Simple Control environments will be 100% separate. All devices will be either V5 or V4.5. The slight exception to that is, because the appliance Hub gets a forced update, it will work in both a V4.5 and V5 environment. All other software will enforce that members of the mesh are the same version as usual. Nothing else is a forced upgrade. Transitioning to V5 is a separate app for iOS, tvOS, and macOS.
Dropbox is also separate. A new app folder is created on Dropbox by V5. You can rename your old one to transition, or keep both.
While you can also install V4.5 and V5 on the same device at the same time, you should ensure as in the past that Background Refresh is only turned on for the app you actually use. The reason to have them both installed at the same time is for instance if your subscription is tied to the old app and you have not yet transitioned it to the new app or the website.