According to GoNintendo, Japanese retailer Enterking posted a warning message on their site, suggesting that your system might be unbootable after a software update if Nintendo detects you've been using an R4.
Japanese wab also told that after reset your N3DS, it also record you had ever used マジコン. The record also let the N3DS was refused to repair or sell(nobody want to buy this product) Japanese also prepare the law for sell マジコン. So if the law is valid in future, may be use マジコン also became illegal in Japan in future.