There was one (infected) inside the fighting compartment and the rest were outside, nowhere in evidence, when they disappeared. Or dragged out of the hatches while unbuttoned. Looked like they got caught away from the tank. Otherwise there's have been four live crewmen, or four bodies, or no one at all inside the tank.
Then again this is Walking Dead, not real life. There were M2s with belts in them in the positions in front of the CDC.
“There were M2s with belts in them in the positions in front of the CDC.”
Perhaps they set the head space incorrectly.*
* Obscure Army reference.