How can I install Facebook on iPhone 4

You probably cannot anymore. Most apps support only the last 3 iOS versions and the current system version is one beyond iOS 18 (it would normally be 19 but Apple just changed its system naming and it is 26). Your iPhone 4 can only run up to iOS 7 which means it is hopelessly outdated and you'll be lucky to find any apps at all that work on it. Even the Apple apps may not work properly (e.g., Safari).

jonatan93 wrote:

big spyware in the third party apps

Odd. I haven't heard of any "spyware" on big third party apps (sure, they all collect marketing data on you, but that's not spyware), and definitely not on the Apple platform. I bet all the media would be full of it. In most places Apple makes it hard to get spyware on an iPhone even if you try very hard.

In your situation with an ancient iPhone it is basically few developers (and certainly not a big one like Facebook) thinks it is worth maintaining old software against security risks or having Apple keep old versions on the App Store. In fact keeping an old, out of date version poses a security risk to the developer's interfacing system. With my bank, if you aren't running one of the last 3 iOS versions and you can't update the apps then you lose mobile access.

I have a 40 year old car and forget about finding parts for it. I asked about a part recently and the parts store said they had three in the USA and they would have to have one sent across the country. I would have to pay special shipping for it. For the other part there were none available anywhere. It just isn't worth it for them to stock a part where I am likely the only person in the entire state to need that part once every 5 years. No, I don't like it any more than you do, but I am also pragmatic.

I kept my old iPhones and used them as iPod Touches for a while, but even then as the hardware ages you find many apps won't interface with new online services. I now have several (including a 4S and 5s) that are in my recycle box.

Note that it is up to the app developers if they want to provide a version of an app that works on a particular model. Many big companies such as Facebook apparently only want to continue to develop for the last three iOS versions since that covers the majority of users. Likewise, Apple only support system development for a limited time on its old models because most users are using newer equipment. Sure, there are users of old equipment but if that's only a percentage or two of users then it just isn't worth the effort to support those few. There can also be security considerations since old hardware may not support modern security features. Companies can decide where they want to draw the lines and Apple is pretty strict on security. Android and Google can decide what they want to do, but I think you will find few verified situations where Apple has fallen down on security.

From the very bottom of Apple Trade In - Apple :

"Does Apple offer recycling?

Yes. Apple Trade In lets you recycle any Apple device (including devices from Apple-owned brands) at any Apple Store and on Apple for free. That includes your batteries and old electronic products as well as free, on-demand packaging recycling for our commercial, education, and institutional customers. When we receive your device, it will be thoroughly inspected to determine if components can be recycled or reused. Whether recycled or reused, all activities relating to the processing of your device will be managed in an environmentally responsible way."

jonatan93 wrote:

I do not believe in the Apple strict security, modern security does not stop big spyware in the third party apps but rather features that requires hardware acceleration, I can understand.

Not being able to install Facebook on your iPhone 4 has nothing to do with Apple. It is Meta that decides what OS they want to continue and support and for the App Store that is iOS 15.1. The same is true for the Google Play Store where Meta chooses the OS they want to support there too. I believe for Facebook on the Google Play store, they require the device to be running Android 6 or later and that will also vary by device. Android 6 was released 10 years ago, so in comparison your iPhone 4 was released 14 years ago.

Every developer makes the decision on OS support depending on the amount of resources and cost they want to continue to offer support on an ever decreasing market of users that currently use that OS. It is a business decision on their part and nothing more.