4 = set user id, this means that when you execute the file set with this, you are running it as the file owner. so if you wanted your normal users to be able to run a certain prog as super user, you would set this.
2 = group user id, pretty much the same as above but run the executable as the set group instead of the set owner.
1 = sticky bit, if you set this on the directory, the user will be able to write to the directory, but only rename, change, delete the files that he/she uploaded. If you look at your /tmp folder it has the stickly bit set ls -ld /tmp
I just did a quick google and found this site if you wanna read it, it might explain a bit better if i didn't.