@mrfoxtalbot
As far as I can tell, you can actually try to change column number by changing the grid width in “simple-masonry-layout/css/sm-style.css” like this:
…
.sm-grid .grid-sm-boxes-in {
display: block;
float: left;
padding: 8px;
width: 21%; /* instead of the default 33.33% value which delivers 3 columns */
…
