What you can do, for example, is crop the long line into 4 parts (each of 25%), and then join them one under the other (`-append`):
convert source.png -colors 256 -unique-colors -scale 1000% \
-crop 25%x100% -append palette.png
This is not quite what you asked for, but by varying the percentage you can vary the crop point, or you can use a fixed width in pixels, like `-crop 63x100%` to get exactly the example output you provided.