You were able to learn four languages just by hearing them on TV?
So you are saying that you learned to speak four languages listening to them on TV?
1) You were able to learn four languages just by hearing them on TV?
「テレビで聞いていただけで4ヶ国語学んだの?」
just by hearing them (four languages) on TV で「テレビで4ヶ国語を聞いていただけで」
2) So you are saying that you learned to speak four languages listening to them on TV?
「テレビで聞いているだけで4ヶ国語を話せるようになったってわけ?」
So you are saying that~ で「〜というわけ?」というニュアンスです。
ご参考になれば幸いです!