If you read the latest Naruto chapter, you would know that Sakura summoned Katsuyu. Here is an excerpt from the Naruto bible (Narutowikia):
"It is also shown that summon animals can also choose when they can be summoned. In the event that they're busy with something, they can't be summoned, however they can have a substitute enter in their stead"
-This means that Katsuyu was DONE healing the Kages, because Sakura was able to summon her, which means she was no longer busy.
-Katsuyu did not even mention the Kages or seemed alarmed at being summoned, which means that she healed the Kages.
-If the Kages have been healed... where do you think they are? Shouldn't they have been on the field by now?
-Orochimaru only mentioned Tsunade... he didn't mention anything about the other Kage or their condition, which makes me believe that they left already. If that is the case, would they really leave Tsunade to die? Don't think they would.
They're either with Tsunade or almost arriving to the battlefield. I wouldn't expect Orochimaru to acknowledge the Kages along with his old comrade. I'd expect Oro to acknowledge his old comrade first and foremost before the other Kages since he doesn't know them as personally as he knows Tsunade.
Or maybe the 4 kages are well on their way to recovery while Tsunade is old from using up her chakra. That could be why Orochimaru singled her out. I doubt the kages would get saved from the brink of death by Tsunade, only to leave her behind and go about their business.