YOU MUST GRAB: https://www.adafruit.com/product/4292 and/or https://www.adafruit.com/product/4295 ; 1gb and 2gb Raspberry Pi 4 at Adafruit, 1 per customer and grab them while they last. Get the Power supply, fan, and anything else that catches your eye there too; Adafruit are amazing people.
Once I got mine, one slid under snafu as a drop in replacement, with no trouble at all. Just unplug the Raspberry Pi 3b+ I built it on and put the SD card into the Pi 4, plug the drives and lan back in.
The bandwidth advantages of a Pi4 for a fileserver really would have justified the expense when I was building the server, but with the price here theres no reason not to.