I have an old Raspberry PI3 (with SSD) that runs DietPI, Pi-Hole and Wireguard. It seems to work fine and never been a problem.
Is there any point in upgrading it to a PI4? PI3 seems to run all the latest versions, but will I see a speed boost with an upgrade? Load always seems to be zero.
I know I’ve swapped out old routers and been shocked at the speed boost. Is a PI4 any faster at doing DNS lookups compared to a PI3?
There should be no need to upgrade. I guess your system is idling most of the time.
It idles doing nothing. Just sits there on top of the router not getting warm. If all it has to do is the odd lookup I can’t see it being overloaded. So I’ll use the PI4’s for other projects.
If I wanted to experiment, could I just shut down and swap the SSD to a PI4? And then swap it back again a few days later? Does the OS even care what hardware it is running on?
Theoretically, you can do this as long as USB boot is already enabled on the RPi4.
If you plan to stay on the RPI 4 longer, you would need to install the eeprom packages from our configuration menu.
This is one of those “lets see if I can tell the difference” tests. I’d backup before anyway, but sometimes you just have to break things to learn something. 
Ideally I want to keep the old PI3 in place.