I didn't want to use scaled down images without adjusting the lower resolution images to be crisper and cleaner. Scaled images often yield crappy results. See this article by John Hicks for more info.
I am using png2ico installed via Homebrew.
Prior to running the conversion I ran the images through ImageOptim (there may be a better way to do this).
Running this command in the directory where the files lived yielded my favicon.ico
png2ico favicon.ico favicon-16.png favicon-32.png favicon-48.png favicon-64.png
##Notes
- I had tried using ImageMagick on terminal to create the .ico file but the filesize was 32KB which was 4x the sum of the individual files.