Local Multiplayer works fine over regionfour, but I was unable to get Taiko no Tatsujin's download play to operate while running it under regionfour on a US n3dsxl. It threw an error message when trying to open the download play screen, while it didn't throw anything when running on my jpn region n3ds.
On top of this, download play is region locked along with the games, so I wouldn't expect it to work even if it did open. Even though the game is running under regionfour, it's most likely going to continue sending out a download copy of whatever in the region the game is from instead of the region the 3ds running the game is from.