Hi there Michael, welcome to the forum.
GTPro4 is a 32bit application so it won't be able to run 64bit plugins I'm afraid.
However, I'm not sure if there is a 64 bit version of Guitar Rig 3 bundled with GTPro4 - I think it's just 32 bit, but I could be wrong.
Which brings me to the question - have you run Guitar Rig in standalone mode yet? I believe there was a problem reported that you have to run it in standalone mode and register it before it will work as a plug-in.
Also, there is definitely an issue in Cakewalk SONAR running the newer version - Guitar Rig 4 LE - before it will work, it has to be updated to the latest update version. I'd suggest you check on the Native Instruments website and make sure you have the latest version of Guitar Rig 3.
If you are going to be using amp modellers such as Guitar Rig inside GTPro4, I'd also recommend you download the free version of IK Multimedia's excellent
Amplitube 3 software - click on
THIS LINK to go straight to the information and downloads page.
Hope that helps, and just get straight back to us if you need any more advice Michael.
Steve