The two piece clip set is MUCH easier to use than the 4-piece...the 4 piecers have little tabs you have to tuck up under the tank lip then snap the clip onto. What a PITA!! One of my sets, the little tab keeps falling out of the tank lip and it's VERY annoying! MUCH easier to just bend the clip and snap it on.

With Oreo, when she escaped, I went from two on the lid, to TWO PER (long) SIDE and now there isn't ANY problem.