Are there any schema design changes that can take a design from 4NF into a different 4NF design? My thought/contention is that once a database schema is in 4NF it by definition can only be changed into something less than 4NF (Denormalisation, 3NF etc) and that there is no alternative 4NF design for it to take.

That is, once a design is in 4NF there is no design choice/change that will take it to some other different 4NF design.

Is this correct (or are there any cases where 2 different 4NF designs are put up for comparison or converted between)?
I dont think so except for the fact that there is a  5NF, which describes a design where your joins are only on the candidate keys.

Many "4NF" designs meet this criteria, but not all, and it is definitely something you can change a 4NF "into" to be be more normalized.