A four space extension, formed by two single stones, is not very common. Usually, extensions of more than three spaces are made from corner enclosures and walls. An interesting feature of these extensions, is that the opponent has no obvious invasion point: the number in between is even. We can distinguish the following examples:
The unmarked extension is a low four space extension.
The circled one is a high four space extension.
The squared extension is a mixed four space extension. The invasion of this configuration is an interesting case: mixed four space extension, invasion
Alex Weldon: It's true that these extensions are usually not seen very often in regular circumstances, but there are a few exceptions. One common occurence of the low four-space extension in modern play is in the mini-Chinese fuseki, shown here. Can anyone think of a joseki that includes/ends with a four-space extension?