I think it's largely about presentation, 4-ch looks like a lonely, static page from 1996.

If I had unlimited free time to devote to this then here's my radical ideas for what I would do different:

• no predefined forums.
• When you post a new thread you give it 3 tags. Each tag is kinda like a forum. As you type, possible completions for each tag appear, aswell as suggested alternative tags that have more threads posted.
• Front page shows most popular tags & latest threads posted in each.
• Make the whole site very dynamic looking, encouraging jumping from place to place on the site. (For example a sidebar with random/newest posts might be displayed on every page)
• Remove the "toy" feature of tripcodes, replace it with the more practical OpenID for those users who opt to identify themselves.
• Along the same lines, turn the "sage" feature into something intuitive, say a "Stealth post" checkbox. This is a genuinely useful feature but badly marketed.

Of course with all these changes, it wouldn't be the same as 4-ch. But that's kinda the point, I think such a site would (unlike this one) have a chance of becoming popular.