Hints: Four of a Kind

(highlight to view)

1. Identify the pictures.
2. They are in alphabetical order.
3. The identifying words all contain the same number of letters.
4. Look for commonalities among sets of the words.

Big Hint: Group the words into four sets of four, using the pattern given: "1xxx, x2xx, xx3x, xxx4", then read the answer as "1234".