>>1
If you mean the software that powers the imageboards. Well you can't. It's proprietary, meaning they're not going to give away the script to anyone, it's closed source.
What you can do, however, is if you take your time learning a programming language (In this case PHP) you can modify futallaby
(The code that 4chan's script is based upon) and make it more like 4chan's script.
(
http://www.1chan.net/futallaby/)
The software that runs this BBS is called Shiichan and you can download that from
http://wakaba.c3.cx/shii/shiichan although you'll have to modify this one as well to get it to behave like 4chan's version.
I did see a download for THIS version of shiichan modifed and everything on rapidshare once but I can't seem to find the link anywhere.
If you can't be bothered to modify all that there is Serissa (
http://www.serissa.org/) and it's based off of Kusaba. 7chan runs this and it makes for a decent 4chan clone.
But, honestly the best out of all these that I recommend is Wakaba (
http://wakaba.c3.cx/s/web/wakaba_kareha.html) unlike all the other scripts which are coded in PHP, Wakaba is perl.
Hope this helps.