What does (4) mean in hd (4) in Terminal Help?
 

Am probably overlooking it, but I have not found what the (4) means (or what can be done with it)in the
result of terminal command man -k hd:

hd (4) - MFM/IDE hard disk devices

The numbers are for different categories of system functions. Some programs or commands have more than one man page available depending on what functions they cover. Most of the time you won't need to worry about them. The default page will cover most needs. If you need to see a specific numbered page, just type "man 4 hd", or such.

Wikipedia has a rundown of the categories: http://en.wikipedia.org/wiki/Man_pages

Thanks for info.