All that extra processing power for $20.00 more.
It's not just the processing power. Everything is nowadays written for RPi 4. So with $20 extra, you're also included into implementations of projects that have more support. I could have an issue with my RPi 3, but if you've never used it, you'll just recommend me to buy the latest Pi.Easier to work with in general, e.g. apt updates take forever on Pis (at least anything up to including 3B+).
Running a full node on a RPi does, indeed, have this downside. 
I wanted to try some different implementations of Bitcoin node OS, but they'll all take around 2-3 months to complete...