All in their primes or best forms at featherweight, I think Marquez would take it with close, but clear victories over Morales, Pacquiao, and Barrera. Are you referring to Israel Vazquez or Wilfredo Vasquez? I assume Wilfredo since he was a featherweight title holder.
Well, we know he loses at least one of them, that being to Pacquiao. Marquez and Barrera you can't say that about in their primes, and they are about even head to head with each other. So, I'd say Barrera is least likely to win this tourney actually, aside from Vasquez.
A more interesting addition would be Naseem Hamed instead of Vasquez. But even then, I think he'd lose to Barrera every time out and get a lesson from Marquez.
I'd take Morales to win against Pacquiao and be about 50/50 with Marquez. I may slightly Barrera over him based on their encounters(you'd have to), but he's capable of winning 4/4. Barrera is capable of winning about 2 or 3 out of 4, as he'll lose to both Marquez and Pacquao, Marque being a closer call prime for prime.