If you flat.. do you think its very likely he'll bluff the river? Is he bluffy aggressive, or just plays strong hands aggressively?

If he's very likely to bluff, I don't mind a flat, jam a non-heart river and flat a heart river (cause I'd expect him to bluff hearts sometimes too).

If he's not likely to bluff then I'd just 4bet since I don't expect this type to be able to fold many strong hands here and there's way more overpair combos and set combos that we beat than combos of 99.