Answer
$Co^{4+}$ is smaller.
Work Step by Step
$Co^{4+}$ has one less electron than $Co^{3+}$. The positive charge in the nucleus is larger in $Co^{4+}$. The greater force of attraction in the nucleus makes the $Co^{4+}$ ion smaller.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.