4/6 & 5/6 refers to the amount of time per second (nominal) that the horn is sounding. Here’s a video I just made to make it clearer.
And here’s a link to the document I mentioned. (Sadly their doc for their photo IC doesn’t have the figure illustrating the patterns)
http://cache.freescale.com/files/analog/doc/data_sheet/MC145018.pdf Page 6.