Is there any Nintendo Promotion going on for Etrian Odessy IV for 3DS

There was not long ago, you just missed it. Both EOIV and EO:U were $19.99 on the E-Shop but the promotion ended on the 19th. It'll probably be a bit until the next one rolls around.

There's like a sale every month or two on the eShop that puts them for $20.
Lately there was a recent promotion that put 3 Atlus games for $15 so hopefully the next time EO4 goes on sale it'll be that low.

Also great choice, EO4 is my favorite 3DS RPG.