I'm on the phone so i can't give a visul effect.

My main suggestion was to add a numeric value (of each index) to their respective previews.

Search for Orgrimmar Grunt (3000 something i think) they have 4 previews. (so using all the indexes)

Having 0 - 3 index number on top or below thier preview just to easier association.

Another suggestion since we only have use up to 4 total indes can we make the preview zone even?

1 Row = 4 Columns [Preview0, 1, 2, 3]
or
2 Rows = 2 Columns (each)
[Preview0, 1]
[Preview2, 3]