I agree with the above. Coincidentally, I also disagree with the OP, but as the thread is no longer about this I guess we'll have to leave it at that :p
Aaaanyway, during a brief conversation on the 4chan IRC channel prior to my banning from said channel (for unrelated reasons), I was told the reason for the 404 was "Btcentralplus" blocking it. This is BT internet's central server, which means they have indeed blocked /b/ access to its customers ('transparently' too, apparently).
According to stories like these:
http://news.bbc.co.uk/1/hi/technology/4689386.stm
http://news.bbc.co.uk/1/hi/technology/4354065.stm
http://news.bbc.co.uk/1/hi/technology/3797563.stm
http://news.bbc.co.uk/1/hi/technology/3786527.stm
They are using a system called "Cleanfeed" to filter and block certain sites according to a 'blacklist' provided by the IWF.
[ IWF homepage: http://www.iwf.org.uk/ ]
The only people that know the internal workings of Cleanfeed are LINX, but BT assure us that the system is only intended to filter out CP. Presumably somebody reported 4chan's /b/ to the IWF as a CP containing site, which then got it put on their "URL database", to be blocked by Cleanfeed. The only people that can view the IWF's URL database are 'members' of the organisation however. Membership costs between £500 - £20,000 depending on the size of your telecomm/internet company.
Apparently the IWF has been known to make mistakes with the sites it blacklists, and there is a complaint/appeal procedure to get mistakenly listed sites removed. Considering as how it is intended that by 2007, -ALL- UK ISPs will by law have to use a Cleanfeed-type system, appealing will soon be the only way that British /b/tards can hope to get to the board at all.