My own two cents: once software updates stop, you need a new phone.
I completely understand squeezing every bit of value out of a phone. But your phone has gps, camera and pictures,microphone, browsing history, text messages, emails, potentially has passwords and financial info, is often inside your home network, more. This sensitive data and access remains secure based on ongoing sw patching of vulnerabilities. The enormous amount of internet ‘bad guys’ are constantly and without stop looking for new vulnerabilities. Don’t make this easy for them.
If money is tight, look to the secondary market for an iPhone 11. I’ve seen renewed in the upper 200$ range. It’ll give security for 4 years and the features compared to the 4s will be VCR compared to 4K. BOL