The Act III series of puzzles was designed to look just like the puzzle page of the Sydney Morning Herald. Each Part was distinguished by the numbers labelling each sub-puzzle (which are used to represent the running total of each puzzle type in the SMH's puzzle page).
In order to make this set of puzzles look similar enough to the puzzle page Sydneysiders are familiar with, some liberties were taken with our usually tight rules regarding extraneous red herrings, but hopefully none of these (namely the puzzle instructions and comments on past and future solutions) tripped anyone up.
Finally, to give a reason for tying these four puzzles together, the solutions to each consecutive Part were deliberately made to relate to each other somehow. These relationships are explained at the end of each appropriate solution page, so don't read past the answer extraction step if you don't wish to be spoiled on the other Parts!
Part 4 here referred to the puzzles labelled with a 4, namely the Quick and Cryptic Crosswords.
Clearly the first step is to fill both these crossword out. The main contributing factor to this puzzle's difficulty and length was in the difficulty of the cryptic clues. See below for an explanation for each cryptic clue.
Once both grids are filled out, they should look like this:
Next we hope to find a hidden message somewhere telling us how to extract the answer. The most common way to hide a message in a crossword is in the unchecked cells, but there doesn't seem to be anything hidden in this way. After a little more investigation however, one should come across a message in the main diagonals: reading the cryptic's and then the quick's gives the message ROTATE AND OVERLAP. Incidentally, although not numbered with a 4, the Target solution was another hint to check the crosswords' diagonals.
So it follows that we want to overlap the crosswords and rotate them somehow. Rotating both at the same time likely won't do much, and at any rate we don't even know which direction to turn the grids. However, as is usual in most crossword grids, these grids have rotational symmetry, meaning that the grid shape will be the same whether we rotate it 90 degrees clockwise or anticlockwise. It thus seems that the grid shape is more important than the letters on it for this step (hence eliminating any ideas of looking for matching letters after overlap).
So if we cut out a copy of the empty grid, rotate it 90 degrees, and place it over either of our completed grids, what should happen? Assumedly we want to extract letters from the completed grids using the empty grid somehow, and to this end there are only so many methods that can be used. One of the most likely methods is to interpret the dark cells in the empty grid as holes, and to thus treat the overlapping empty grid as a sort of sieve through which we can select letters that are revealed when its dark cells are cut out. That this seems the right approach is supported by the fact that, looking only at those dark cells that don't lie on even coordinates, there is never a case after overlapping and rotating that two such dark cells overlap - i.e., every artificially-added dark cell will overlap a letter when the top grid is rotated.
The above admittedly confusing description is far better understood when paired with the resultant images. If we cut out a clean copy of the crossword grid, cut out its dark cells, rotate it, and place it over first the filled-out cryptic grid, and then the filled-out quick, the following letters are revealed:
Reading left to right, top to bottom, we find the message DERRICK MACNUTT'S OBSERVER PSEUDONYM. Macnutt was an influential crossword compiler who wrote under several pseudonyms, but the one he used when writing for The Observer was XIMENES, our answer.
The connection with Puzzle Page - Part 3's answer, "Retriever", is that both "Retriever" and "Ximenes" are clued as well-known aliases of people.
The connection with Puzzle Page - Part 1's answer, "Nina", works in two ways. First, mechanically, "Ximenes" is discovered after following two unconventional ninas (where a "nina" in cruciverbalist jargon is a message hidden in a completed crossword grid). Furthermore, just as "Nina" lends her name to a common word used in the context of crosswords, so too is Ximenes' name found in crossword jargon as the adjective "Ximenean", used to describe cryptic crosswords clues that follow Macnutt's list of rules. Amusingly, most of the clues used in this puzzle's cryptic would likely not be considered Ximenean.
The cryptic crossword clues are explained as follows:
(Note the last clue was an &lit., as indicated by the exclamation mark.)
|The answer is: ximenes|