Family 0x0004 is used for messaging. As in, normal instant messages sent to other clients through the server. It's also used for various other things that are all basically one client talking to another client through the server. Examples of "various other things" are file transfer negotiation, direct IM negotiation, RTF messages, and old-style ICQ authorization.
This is all documented either by first hand packet captures or by tracing through libfaim.