xxx is the largest format, divided by period. You cannot use xxxx (when creating with Xcode 6. In older versions of Xcode, the xxxx version format is allowed).
It is possible that the input field in iTunes Connect does not check the input that you provide for this case, so you could enter it there. The problem is that Xcode does not allow you to use it, and since your version string is checked when loading the binary, you must use the xxx format
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