"Is it possible to check the remaining eMMC usable life on a base 4200?"
The correct answer to this question is YES you can, if you know how, and any Linux users out there already should know how.
Anyone that does get a 4200 and desires to utilize its onboard eMMC storage to run it, since FreeBSD cannot currently use mmc-utils on 4200s to be able to see the health stats of eMMC storage like other models can as several Redditors continually try to warn others about, they are only half correct, only when FreeBSD/pfSense is the current OS that is booted and active is it impossible to use mmc-utils to check these stats on 4200 model boxes, not the case at all when Linux is running on it, even the Redmine that got closed that users complain about states this fact.
I highly sugest that 4200 owners ahead of time prepare yourselves a diagnostic USB boot drive using a live-Linux iso image such as Kubuntu or similar to prepare a persistent live linux bootable USB drive that has persistence storage, boot it on a PC after to connect it to the internet and install the Linux verion of mmc-utils which does work for checking eMMC health stats on the 4200. Then when scheduled maintenance time periods come up this diagnostic USB boot drive can be used periodically to check those eMMC stats in a couple minutes doing a quick reboot to check.
A diagnostic boot drive like I describe here can be prepared by any user in under 10 minutes other than the time it takes to download and to flash the ISO to a USB drive which can vary.
It is not impossible to do as many Redditors claim in their posts, you just have to know how, users that are already familiar enough with Linux probably figured this step out from day one of seeing Netgate's closed Redmine that was requesting several hours or days of work to re-write the coding of a ported package that isn't quite so simple.