I am installing a 100 amp pool sub panel for an inground pool. I am running #4 thwn in pvc from the house to the sub panel. My question is can I put a junction box in my basement and convert to #4 romex to the main panel. Question #2 Do I attach the #8 bare bonding wire to the ground bar of the sub panel?

Yes to both questions. Be sure the neutral wire connects only to an insulated neutral bar.