If A == B, then B just is A, we're saying A == A. A possible world where B != A is a world where A != A. You seem to believe that A != B even in our world.
(1) is the essence of the physicalist claim. You can certainly declare it bullshit if you want...
The leap in in (3). While brains and mental states as we know them must be identical, according to physicalists, other worlds may do brain-like and mental-state-like things not as we know them.
For instance, we may have misunderstood our own world.
Oddly, (4) gets back on track. If zombies can be proven to be possible (which conceivability does not do) then physicalism is false. (Although, this generally implies epiphenomenon, which also implies causality is false, which puts consciousness outside of logic {P -> Q} let alone physics.)