I am thinking of buying a new Zone 1 - 4 annual, and then trading in
my Zone 2 - 4 annual. Is this possible, and how much will I get back

You might find they base any refund on the cost of monthly and weekly
tickets - so as though you had been buying monthly tickets. This probably
means you'll pay the one month at last years rate and the rest at this
years and the refund would be less than you expect. This also means had
your ticket been more than 8 months old there would be little refund value.

I came across this when I went into hospital some 8 months into an annual.
The refund for 6 weeks away from work wasn't enough to cover the couple of
months remaining on the ticket when I returned to work. Not very fair.

Regards,
Dave