Alot of us who have a Raspberry Pi 3 or 4 might upgrade to the Raspberry Pi 5.
Doing so would leave us with 2 Pi’s. What are some great use cases for the older Pi, that would no longer be the main machine?
Sell them used and reduce the cost of entry to other people
Here is a controversial thought; do you need to replace the 4?
I can’t speak for the OP, but in my case… It was very much a resounding YES!!! If your using Pis as general home servers, like I was, you can very quickly run into resource constraints. I wound up replacing 5 Pi 4’s with a pair of 4th gen Intel boxes and I’m still hit resource limitations from time to time. Though now it is more io related then ram or cpu.
You didn’t blindly upgrade to the next Pi. Not upgrading or upgrading to something you know meets your demands are the right things to do. I assume upgrading to a Pi 5 is not that for most people