Well, lockdown and being unemployed does make for a pretty empty schedule 😜 

I'd love to find out if we can find a solution though. After all, tossing perfectly good phones because software no longer supports it, is rather ridiculous right?

Things I looked at:

- Upgrade the 4S to iOS 13 anyway

There seems to be a "hack" that allows upgrading to a beta of iOS 13. Granted, it is a beta and I do not even know if it is still available and what kind of issues you may run into.
Worth a consideration; YouTube video, you may have to Google for more info.

- Install a different OS, which seems useless

So far I couldn't find any Linux or Android distro that would work on an iPhone 4S.
I did see support for newer models though, even though these all seem to have one or more limitations, some of them not supporting the camera.
So even if we would find a working OS, it would most likely not support the 4S and even if it would, the camera would most likely not work.

- Find an alternative iOS store

I did only a quick search, and didn't explore any of them in detail (I do not have a 4S).
This article lists quite a few, so these may be worth a look.

AppCake seems promising, and they do list the supported iOS devices.

Note: I'd be careful with any of these stores - any private info should probably be removed from your phone before even considering this.

- Jailbreak / Cydia

I have done jailbreaking in the past (no longer doing this though) and the Cydia App Store offers quite a few fun (and old!) things that may be helpful.
Searching for apps can be a pain though.

- Whited00r

Whited00r is/was (not sure how active this project still is) an alternative iOS version, but it doesn't support the 4S (as far as I can see).
Their forum may provide solutions, or not.

So I guess I was expecting the bad news!!  I wonder if there is a physical way to remove the camera from the phone, and convert it to a USB webcam?

Anyhow, I'll investigate the jailbreak option!

Thanks again Hans.

Welcome to the forum 😊 

Finding apps for an old iPhone can indeed be quite a pain. It is very unfortunate that Apple removes "old" apps, rending old iPhone useless, even though an app did exists previously for it.

I suspect that you'll have to go the jailbreak route because of this.
I've looked and like you, I couldn't find any useful apps for iOS 9 (to run your phone as a webcam)...

If you decide to explore the jailbreak option, which may not be a bad idea considering that your phone will never be updated again, then maybe look ios-repo-updates.com or similar sites that have Cydia packages listed. At ios-repo-updates, searching can be a little bit of a pain though.

Sorry that I do not have a better answer 😊 

There are LOTS of app solutions for using an iPhone as a webcam, but I'm struggling to find one that will work with an old iPhone.

My old phone is a model 4S - model A1387 - running iOS 9.3.6.

Also I will need to use it with a Macbook Air - so that eliminates any Widows solutions!

Why? My wife uses Zoom a lot for work.  I have connected a large monitor to her Macbook which works well.  Ideally, I should just fit a webcam to the Macbook, and mount it over the monitor screen for the Zoom sessions.  But I've tried a couple of webcams and had disappointing results compared to the built in camera in the Macbook.

I've read that the camera in even old iPhones is better than the built in one in the Mac, so I'd really like to use the old model 4S for this

Any help or suggestions would be much appreciated!