currently the latest 3ds firmware is V6.2.0-12U, the latest 3ds firmware has blocked many flashcarts, most flashcarts currently can not work on it. so if you plan to buy a r4 flashcart for your 3ds V6.2.0-12u, make sure buy a flashcart which can work on it. this post will tell you how to choose a r4 flashcart for your 3ds and what is the best r4 flashcart for 3ds V6.2.0-12u.
currently there are only two or three cards can work on the latest 3ds firmware, including r4i gold 3ds, supercard dstwo and r4i sdhc 3ds. they have released the update patch for the new 3ds firmware. so they can work on the latest 3ds, and we should choose the card from them.
the first r4 flashcart i recommend for 3ds V6.2.0-12u is the r4i gold 3ds, it is world's first flashcart that work on 3ds V6.2.0-12, they released the update patch just a few hours after the 3ds new firmware released. r4i gold 3ds card is a very good cart for 3ds V6.2.0-12 and also always can be updated to work on the future 3ds new firmware, which is very important.
make sure the r4i gold 3ds card is this one: R4i Gold 3DS Wood R4i 3DS flashcard For DS DSi 3DS
the second flashcart i recommend is the supercard dstwo, it is one of the best flashcart that work great on 3ds, dstwo today has released the update firmware for the new 3ds new firmware V6.2.0-12u, and with the update patch, you can easily update your dstwo and make it work on 3ds V6.2.0-12u.