Solution: Four of a Kind

(highlight to view)

Each of the 16 images leads to a four-letter word. The 16 words are in alphabetical order:

acne, bulb, dial, glue
hula, kiss, note, pita
salt, test, thor, tool
tray, vane, well, wink

As suggested by "1xxx, x2xx, xx3x, xxx4", the 16 words can be grouped into sets of four, where the first group shares a common 1st letter, the second shares a common 2nd letter, etc. The four groups are:

Test, Thor, Tool, Tray
dIal, kIss, pIta, wInk
buLb, huLa, saLt, weLl
acnE, gluE, notE, vanE

Reading the four common letters in order (bolded above) spells out the four-letter solution, "TILE".