Resurrecting 4000s is now my mission
Sounds like it's working. If the drives spin up etc and then it boots from floppy you're in good shape.
First things first though, if you have not yet removed the battery from your 4000, assuming it's a barrel type battery and not one of the coin style ones, DO IT NOW
! If not before
If you are lucky and the battery hasn't damaged anything, putting power through it might push it over the edge... Really, remove it now.
Regarding monitor, get a different scart cable and, if you're still getting nothing, try using a CRT tv if you have one anywhere. If all that fails, the 520 is a cheap and cheerful option for checking your machine works.