We have compiled and tested Forest 3.3.1 with a beta version of VRay 1.5 SP5, that will be the official VRay version for Max 2011.
According from the ChaosGroup information, you can use SP4 with Max 2011, but its compatibility is not guaranteed. Because the differences between libraries, Forest doesn't recognize this configuration.
As workaround, you can install Forest 3.3.1 for 2010 in max 2011+ VRay SP4. With this configuration, the library versions matches. However, we can provide support for this, and i suggest that you wait to the final release of SP5.