Was the jack on the flop a club? Could be he holds Jc, had top pair on the flop, picked up the J high flush draw on the turn, and wanted to stick around...Or it could be another jack with K/Q/A of clubs kicker that picked up the draw on the turn too...

As long as the river wasnt a 4th club, i'm pretty comfortable with my 4 high flush. I bet 75 cents on the river for value, and hope i don't get re-raised too big.