There will be some major changes, but not as drastic as the Python version upgrade. I'd think the 4.2 book will be totally fine, and when 5 is out, just look at the changelog and start adding what you need from it.
FYI, one of the big changes is in integration tests on controllers... it's not a huge difference, but if you try to copy from the book instead of generating your controller you might have issues. Also only the 5.1 RC is out.
If I was learning, I'd build an app with the current stable version of 4, a toy crud app with 100% test coverage, then try to convert that to 5. Then compare that to building 5.0 using it's generators. You are likely to work with legacy code, so learning 4 is not bad. May well do that myself, so I'm catching gotchas like the integration test changes in my toy and not at work on a deadline.
But yeah, coding, at least for me, is all about trade offs, and as long as you understand the rails trade offs and mitigations, you are fine.
