"Tennis is a game for two or four."
Translation:Teniso estas ludo por du aŭ kvar.
In some languages (such as Japanese), the number standing alone without any decoration could only work in something like, “Blackjack is a game for twenty-one” (i.e., the game is to 21, or 21 is a number the game is about in some way). So I’m not sure that pointing at the literal English is helpful here—it would not allow you to leave out the people-numbering-signifier in languages that require it. But Esperanto isn’t one that requires it, but is one that allows it.
So, does por duopo aŭ kvaropo mean something significantly different than por du aŭ kvar in this case? Like, if it implied that tennis was a game for couples to play cooperatively rather than competitively?