I only know that "よん" is means "four" (kanji: 四)
Some reference is https://en.wikipedia.org/wiki/Puyo_Puyo~n where is also "よ—ん" written.
Words and example:
うわ—い! かったよ—ん! (Source: Kingyo Chuuihou! 2 - Gyopi-chan o Sagase! - in-game conversation, after player winning one of the mini-games Wapiko conversation). Word without "よ—ん" = できたよ / with "よ—ん" = できたよ—ん ; 終わったよ = 終わったよ—ん.