|Solution for 4.1 Final Destination (Answer = BITTERN)|
The stamps are rebus puzzles representing the names of suburbs of Victoria. In order from top to bottom, these are:
The little orange boxes on the envelopes represent the postcodes of these suburbs:
With the except of the leading "3" that represents Victorian postcodes, each column is the starting digit sequence of a mathematical constant or sequence: 31415 = pi, 05772 = Euler's constant and 11235 = Fibonacci sequence. Extrapolate these sequences gives the final postcode (3918) which corresponds to BITTERN.