4-ch (at 4-ch.net) is a web site with discussion boards for various topics, including anime, romance, politics, internet culture, and other standard forum fodder. It is modeled after 2channel (or "2ch" for short), a massive Japanese forum that is highly influential in its parent country and culture.
2ch and, by extension, 4-ch are notable for their completely anonymous nature in posting. It is possible to specify a name and e-mail/web link with each post, but this is generally not done except by moderators or attention whores who think they're cool by trying to make a name for themselves online.
4-ch, by virtue of its origins, tends to attract Japanophiles, but posts in Japanese are mostly relegated to a special board for that purpose. The site is primarily English, with Japanisms mixed in for flavor (facemarks and orz being among the most common).