Apple Watch Fall detection requirements
What does an Apple Series 4 Watch require to have Fall Detection +
Emergency 911 calling fully operational? Does the watch need to be paired with a certain iPhone Model/iOS release?

You only need to meet the minimum iPhone and iOS requirements.*

GPS + Cellular:  iPhone 6 or later running iOS 12 or newer

GPS only:  iPhone 5s or later running iOS 12 or newer.


Obviously, either model will support fall detection.  However, for fall detection to work with the GPS only model, you will need to have your iPhone with you whereas the cellular model, you don't.

Does anything need to be manually configured/enabled on the watch?

Yes.  You need to configure both Emergency SOS and your Medical ID information.  Full details on how/what is involved can be obtained from this Apple Support document, https://support.apple.com/en-us/HT208944
Pay particular attention to these caveats:

https://support.apple.com/guide/watch/make-an-emergency-phone-call-apd4ea933124/watchos


Some cellular networks may not accept an emergency call from your Apple Watch Series 5, Apple Watch SE, or Apple Watch Series 6 if your Apple Watch isn’t activated, if it isn’t compatible with or configured to operate on a particular cellular network, or if isn’t set up for cellular service.


https://www.apple.com/watchos/feature-availability/

The availability lists countries and has a footnote explaining in footnote 2

Device must have been setup for cellular service with a carrier but does not require an active cellular plan. https://support.apple.com/en-us/HT207578

Footnote 1 says series 5 is needed:

The international emergency calling feature requires an Apple Watch Series 5 (GPS + Cellular) model.


*Apple Watch Technical Specifications