As you probably know, part of the OS in Amiga usually is in the Kickstart ROM (some system libraries etc.). With UAE you have some kick.rom image instead, usually the version can be identified by it's filename. You can probably check the version from early start-up menu too and of course from the chips directly if they still have some stickers on them (
Kickstart information at
TBBOAH)
If you have Kickstart 3.0 chips on your A4000, easiest route would be using Workbench 3.0 (AmigaOS3.0) with it. However if you have Kickstart 3.1 chips in it, you can use without too much hassle AmigaOS3.1, 3.5 or 3.9.
Combining those and with some startup utilities depending on different versions of system libraries could very well been the reason for that "invalid version" issue you had some time ago.
