Was quite easy actually, after the upgrade MIDI out has been added to GA4.:
Open the Rack Instrument (hit F11) and add both BFD and GA4. (I do not have BFD but did this with Superior Drummer and I guess it is the same thing with BFD)
Be sure that MIDI out is on in GA4. You will find the symbol above the pads and above the transport counter. It is a MIDI symbol with a small x under it. The x should not be yellow.
Load the set and/or groove of your desire in GA4.
Load a drumset in BFD. On the BFD track, in the inspector (far left in the project window), go to where it normally says “All MIDI Inputs” or the name of your MIDI input device. Click there and change that to Groove Agent and the corresponding Kit number.
Be sure that it is the BFD track that is the active track. Click with the mouse on the pads of GA4. You should now hear the groove played by both sets at the same time. Go to the mixer and drag the fader of GA4 all the way down and you will only hear BFD. (I am sure there must be a better way to turn off the GA4 sound, but this method works. You should now be able to use all your BFD kits). However: Most kits from different companies have their own mapping. So you may need to remap BFD. I needed to remap Superior. Remember that you have turned off your keyboard (or MIDI device) for BFD so you need to use the mouse to click on the Pads. Or if you set both tracks to active you can use the keyboard to trigger Pads or single hits. The signal will then go through GA4 and use the sound of your BFD.