Is the API version different from the client version?
There are multiple versions of Forge for each Minecraft version, because they update it when bugs are found or when adding features. You are probably trying to use a mod that requires a development version of Forge for Minecraft 1.4.7.
To illustrate, Forge 6.6.0 is the latest stable version of Forge for MC 1.4.7, but the very most recent development snapshot available from the Forge file server is:
Build 6.6.2.534: 2013-02-24 23:03:06 for MC: 1.4.7