Construction-wise, there's no difference whatsoever - a 4-wire pickup simply has all four coil wires available, while a 2-wire has the coils jumpered internally. The extra two wires allow you to have switching options, like series parallel, and coil splits, or they can simply be tied together for standard humbucker functionality.
No, they're drop-in replacements - they'll just have to be wired up slightly differently.
but is there a case where a guitar was originally configured for 4 conductor wiring and then a humbucker with 2 conductor wiring will not fit in?
if the above is not true, then will 2 conductor wired humbuckers work as replacement in any humbucker guitar?