I think he's just going to have 6x the vast majority of the time here. Not necessarily easy to figure out in-game though.
One reason I think its more a lot more value than bluff is that we have the Q
which cuts out a good amount of his flush draws that could have check called flop. The Q
doesn't cut out all of his flush draws, but, flush draws that check/called flop feel more natural as checks on the turn than a donk lead ... they can either see river for free or consider a check/raise turn bluff. This seems more intuitive and profitable than trying to construct turn lead ranges here. I'm not saying he won't ever lead turn with a flush draw I just think it can be discounted a good amount.
Then... what other bluffs does he have on the turn? Gutshot straight draws on the flop I think would often check/raise rather than check/call flop to donk lead turn. Villain cant count on such a card falling on the turn, and most other cards won't make sense for him to begin leading out on as a bluff.
If he did have 6x, its easy to see why he might lead turn. This isn't a good card for us so we will often check back. 6x will want two more streets of value and he could just decide he needs to take the lead now to get max value.
Also, if you fold on the 4 river, you should try to make your decision on the turn: either fold turn or call turn + river.