@SylverReZ, I think I have a 256GB card in my 3DS and it's mostly full but I have emulators and tons of other things on it, plus like 20-30 3DS games and it's about everything I would want on it.
With 4TBs I'm pretty sure even if you put everything on it that it was able to run I seriously doubt you could even fill it half way IF a 3DS could even see all of it lol
According to Google AI lol Nintendo 3DS game cards can range in size from 1 to 8 gigabytes, with 2 GB of game data at launch. The combined size of all 1,547 3DS titles is 267 GB