It really wasn't until I got myself a Pi 4 that I really appreciated what an improvement it was over the earlier generations. I have the Pi 4 and a 3B+ running BOINC, crunching away on World Community Grid[1], and the 4 is at least twice as fast at completing work units (it's too early for RAC numbers to settle down, yet). The Cortex-A72 is a huge step up from the A53. [ETA: both are actively cooled.]

Also, the Pi 4 eliminates the USB2 bottleneck the old Pis have, and has a couple of USB3 ports.

[1] Be sure to boot with arm_64bit=1 in config.txt or you will get no work units.