Here’s a Raspberry Pi 4 in a NAS kit with 4 x 2.5" disks, in a RAID5, with EXT4. He got a speed of 90-100 MB/sec, which I consider impressive.
Edit: there’s no distributor in North America yet for this NAS kit. Only Asia and Europe. Also, I’m not convinced yet that the drivers are mature enough to handle heavy loads, and other torturous real-world NAS usage. The disk controllers are J Microns, that’s good, but I have no sense yet for the reputations for those particular chips (as supported in the Linux Kernel currently shipped with Raspbian): JMS561’s.
Edit 2: I asked about the JMS561 more here: https://www.raspberrypi.org/forums/viewtopic.php?f=36&t=266542