What will it cost for 4 adults to charter a vehical to go from Jiuzhaigou to Songpan via Huanglong?

It is not too hard, so please do not worry and you can be in Songpan by the end of the day.
For a 4 seat car, the fare is around 400 to 500RMB for your trip.

Thanks! Now I have three other questions. First, can I be sure of getting four seats on a bus to Dujiangyan from Songpan in mid-October? Second, what is it like in Dujiangyan right now? I can't find anything online that tells me if I should expect to be able to see the tourist sights there in October or whether things will be a mess. Third, has the road south from Songpan been so badly affected that it will be hard to travel it in October?

If you want to get a car from Songpan to Dujiangyan, it is suggested to book in advance and yo can turn to get help from the hotel you live in Songpan. In this way, it is sure you can get a car. Or from Songpan Bus Station, you can find direct buses to reach Dujiangyan.
Things will turn better in October and the local government try their best to recover the damages.
It will not be so bad during October, so please do not worry about it.

Thanks again, Coral. Your answer is very helpful. Since you speak of getting a car, I have one more question. Do you or any other reader know how much it would cost to rent a minibus, SUV or car that will COMFORTABLY adults, to go from Songpan to Dujiangyan?