Will these lights fit on Trex deck posts I previously bought caps that were supposed to be universal but will not fit my Trex posts

This post cap will fit on a post that measures 3.5-in x 3.5-in, 4-in x 4-in, 4-1/4-in x 4-1/4-in and 4-1/2-in x 4-1/2-in. It will not fit on any other sized post.