Dick Schmidt has done testing that seems to show that its a 4x10 if notched through the top edge as shown.

If something more like a tusk tenon is used then the void is closer to the neutral axis and its done less damage to the beam. There isn't the notch in the tension edge of the joist either. Assembly just became a bear.

This is a real rough sketch, I'll leave it to the others with more experience as to whether this has any merit;