The way I read it, it is pretty straight forward.
The dragon, as a Standard Action, can make 4 claw attacks and a Bite attack.
Nowhere does it say those 4 claw attacks all need to be on the same target. Therefore they can be against whoever the Dragon wishes.
The Bite attack can then be made against any other target who was not attacked by the claws.
As far as the fighter's (or other defender's) marks, any attack (claw or bite) not targeted at them will take the mark penalty (and associated Combat Challenge etc).
The thing to remember is that this is not ONE Attack with Multiple Targets, but FIVE attacks, each with their own targets. And so they resolve as separate attacks.
So yes, no matter what, the Fighter gets one Combat Challenge attack against the Dragon. Might just be better to not target the fighter at all, and take down that squishy standing next to him.
I think the confusion comes from trying to imply too much. I have found generally very little need to imply anything in 4th Edition, especially Monster Stat Blocks. What you can and can't do are usually called out pretty clearly.
Besides, making a bite attack against 'a different target' does not at all imply the 4 claw attacks all need to be made against the same target. Just that the bite attack needs to be made against a target not targeted by a claw attack.
The dragon, as a Standard Action, can make 4 claw attacks and a Bite attack.
Nowhere does it say those 4 claw attacks all need to be on the same target. Therefore they can be against whoever the Dragon wishes.
The Bite attack can then be made against any other target who was not attacked by the claws.
As far as the fighter's (or other defender's) marks, any attack (claw or bite) not targeted at them will take the mark penalty (and associated Combat Challenge etc).
The thing to remember is that this is not ONE Attack with Multiple Targets, but FIVE attacks, each with their own targets. And so they resolve as separate attacks.
So yes, no matter what, the Fighter gets one Combat Challenge attack against the Dragon. Might just be better to not target the fighter at all, and take down that squishy standing next to him.

I think the confusion comes from trying to imply too much. I have found generally very little need to imply anything in 4th Edition, especially Monster Stat Blocks. What you can and can't do are usually called out pretty clearly.
Besides, making a bite attack against 'a different target' does not at all imply the 4 claw attacks all need to be made against the same target. Just that the bite attack needs to be made against a target not targeted by a claw attack.