[n.s.]

The only reason I can think of to continue is that PJ knows that we didn't flop a big hand (including an OESFD) because we didn't check-raise or lead out on that draw-heavy flop. Also, he knows that we wouldn't call the flop with 66. So, the only hands that make sense for hero are J6/T6/36, and if PJ doesn't know that OP read stox's book (or never read it himself), then he might think that OP wouldn't call PF with any of those hands.

So it's possible that PJ went through that thought process and determined that OP simply couldn't have a hand that beats his overpair. I think that's enough doubt to call here and call again on the river (plus we could have outs again JTs).

I think 4-betting is too much because too often PJ will have JJ or TT here. Also it'll be an easy fold for him with an overpair when he'd probably bet again on the river with it (so we win the same, lose less).