2015, Vladimir Lepetic, Principles of Mathematics: A Primer (in English), John Wiley & Sons, →ISBN, page 399:
Similarly, if one can prove that there is no one-to-one correspondence between the groups, manifestly they are not isomorphic. For instance, each group of fourth order is either isomorphic to the Klein group V or to the group of square rotation R4, while V≇R4.