Be careful though, IBM 4381 is an example of the mainframe world, so very much not similar to IBM i. It's s/370 compatible and ran OS/VS1 and VM/370.
In terms of abstraction, almost the opposite in some sense, as I've noted in my last paragraph. In terms of usage as well: IBM i's command line model I also mentioned helps a lot in using the system even without external documentation (more than the UNIX shell does), which seems to be the opposite in the mainframe world.
Yes, that's true it is a completely different beast from IBM i. By the way, I'm not sure if you have just used these remote or virtualized or in person but they are quite impressive from a hardware perspective and built incredibly solid compared to almost everything else that I've worked with including VAXen from that era. We had two of them (cold spare...) maxed out Group 2 models.
That's still 'only' 32 MB of RAM which may seem tiny by today's standards but that machine happily served a few hundred branch offices of a fairly major bank all by its lonesome, so that's 1000's of concurrent users (https://en.wikipedia.org/wiki/CICS).
In terms of abstraction, almost the opposite in some sense, as I've noted in my last paragraph. In terms of usage as well: IBM i's command line model I also mentioned helps a lot in using the system even without external documentation (more than the UNIX shell does), which seems to be the opposite in the mainframe world.