You did not self-teach yourself 4 languages to fluency at age 16-18. You might have taught yourself 1 to a level of fluency, but even that would have taken at least several years of hours per day of work.

I'm not trying to diminish your success, but claims like those are laughable. Fluency is a C1/C2 level on the CEFR. No way have you done that for 4 languages so early in your life. And further, C2 (fluency) is around twice as hard as reaching B2, which is the level that Language AP Exams and SAT IIs are. So, unless you can make a perfect score on 4 self-taught language exams, and know those languages around twice as well as the test can prove, then seriously shy away from claiming such a level of fluency.

Regarding it being an EC, I would not mention it too much unless you can show proof of your achievement in the self-studied languages. Personally, I'm taking the SAT II for Chinese to show proof of my 1.5 years of self study. Otherwise, I would not discuss it as it is not too impressive without any proof.