Zuikaku remained available after Coral Sea, along with substantial surviving aircrew from both Carrier Division Five ships, but doctrine discouraged combining incomplete groups. Preserving that rule was inconsistent with drawing instructors from the Yokosuka air establishment to fill other gaps. Kido Butai sailed with nearly 250 aircraft when the surviving groups aboard Zuikaku might have raised the force above 300. A fifth carrier would have added strike weight, scouts, fighters, another flight deck, and defensive depth to an already underprotected force. Even relatively weak Japanese anti-aircraft guns would have mattered in greater numbers, while extra ships could have drawn attacks away from the carriers. Leaving Zuikaku behind reduced both offensive reach and the number of targets the American attacks had to divide among.
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Leaving Zuikaku behind while breaking doctrine elsewhere
Japan declined to combine surviving Shokaku and Zuikaku aircrew on one carrier even as it disrupted training establishments to reinforce other air groups.