I am in fact all ears! I was nonresponsive for a while because I got unexpectedly busy with other things and then fell very ill. I'm still bedridden and running a fever, so if what I write here is not entirely lucid, please cut me some slack.
Regarding unaccounted-for doubt, I should have made it clear that I'm a pending Windows refugee, have only recently started using Linux, and can fairly be called a "nontechnical user." I have no background in coding or network administration, limited, second-hand familiarity with Linux file systems, and no experience whatsoever with NAS. I've assigned static IP addresses in my router, set up some firewall rules using Gufw, and attempted to sync the data on my Windows laptop and Linux laptops using Samba -- which I subsequently learned is outright defective in the Ubuntu-18.04-based distros I'm currently using! -- and that's about the extent of my technical prowess. Feel free to consider me a clueless noob.
I appreciate all of the feedback I've received in this thread. I'm reassured to learn that there should be no significant hardware bottlenecks with the Pi4B, other than possibly/probably a four-USB-device cap (which shouldn't be a problem for me).
Having experienced both the IBM "Deathstar" and WD "click of death" fiascos first hand, I would never rely on a single drive. For some twenty years, I've had two internal drives in my computers and used an "initial clone + daily differential clone" redundancy strategy in 32-bit Windows XP, using xxclone, and a "monthly clone + interim real-time data-file/config-file sync" redundancy strategy in 64-bit Windows 7, using Macrium Reflect and FreeFileSync/RealTimeSync. I've had system drives suffer mechanical failure or irreparable corruption several times, but in all that time I've never had to reinstall and reconfigure my OS and apps from scratch, and I haven't lost any data. Moreover, I was able to get back up and running in a matter of minutes. (I have configured FreeFileSync to create an age- and number-capped archive of timestamped versioned backups of selected files when they get overwritten or deleted by a sync operation. This allows me to recover from file corruption, editing screw-ups, and unwanted deletions -- each of which has happened to me more than once.)
For external backup, I've been -- depending on folder contents -- mirroring, updating, or syncing my local data and configuration files to two external USB drives. (I sync my local versioned-backup archives, as well.) This has become very tedious, which is a leading reason I'm interested in setting up an NAS. The other leading reason is that my data has grown to the point that I'm approaching the reasonable capacity of a 2TB internal drive and would like to offload my big data files (e.g., videos) to external media that is "always" accessible from all of my computers and that is reasonably fast to write to and read from.
I'm definitely going to want drive redundancy in my NAS, same as for my system drives. If I use OpenMediaVault's software RAID 1 mirroring, I'd get instantaneous redundancy but also instantaneous replication of corruption. Moreover, there is a non-negligible write-speed penalty (~15MBs?) compared to writing to a single unmirrored drive. And finally, in case my Pi4B NAS dies or gets corrupted, I insist on being able to to pull either drive, directly attach it to any Linux computer, and read from and write to it without additional hurdles. I'm guessing this should be possible (with ext4, at least), but it's something I would need to nail down before even considering a software RAID 1 mirror. I think it's more likely that I would run my NAS drives as JBOD and periodically rsync the primary drive to the secondary one. I would not have instantaneous redundancy for the most recently modified/added files, but I would have a window of opportunity to intervene if I notice that something is wrong on the primary NAS drive.
NOTE regarding ext4 vs. XFS vs. ZFS: My biggest data file is under 3GB, and the only time I'm going to be writing large numbers of biggish files (videos, for the most part) over an extended period of time is during the initial offload from computer to NAS. I'm not sure whether XFS presents a big advantage over ext4, which is more universally supported out of the box and which supposedly recovers better from power failures as well. (My NAS will be attached to the same UPS/surge-protector I use for my modem and router, but as with hard drives, I've learned not to count on UPSes' being 100% reliable.) As for ZFS, I'm under the impression that it's not possible to just pull a drive, directly attach it to a computer, and use it. (Also, doesn't it eat up a lot of RAM?) If I'm wrong, please let me know.
At any rate, I think my most important questions have been answered. The Pi4B will be able to handle what I want to do hardware-wise. Thanks very much to everyone who took the time to reply -- I'm grateful and apologize that I wasn't able to check back in and say so earlier.