If you have customized a forum skin for you current v3.x version of SPF, I would recommend that you at least download a beta copy of the forum plugin and familiarize yourself with the changelog readme file included. As its a major version with many new features and updates, the css required for a forum skin has also changed.
It is not a major redesign of the general skin appearance, but user customized skins will need to be modified to support the new features. Also, we have applied some extra styling to existing features that formerly had none defoned.
By downloading one of the beta version before the formal release, you have some time to get your skin ready. As always, if you have any issues or questions, just drop us a note in the support forum.