Only going by observation and use of Expert Flyer.
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4A...4B..4C
Qantas.com/yourbooking seat selection will show the 4B seat as available to you. Another WP would see it as unavailable. straitman and I tested this out back in February when we were on the same flight.
When travelling on 737 with SWMBO, and there's a row of three seats free, I'll preallocated a B seat for me, put her in the C seat and the A seat becomes blocked*. Over the last 12 months, the window seat has remained free 75% of the time. 
* or E, D & F as the case may be.
