As is traditionally the case, the Monster Hunter games employ their own added savegame encryption on top of the game system's native encryption, which is in this case the Nintendo 3DS. So if you can somehow get a decrypted MH4U save off your 3DS, you still need to remove the game's own save encryption. This is what these two scripts are for! I'm leaning heavily on svanheulen's
work with this, but if you don't want to install Python just for Monster Hunter save games, then you're in luck.
The MH4U saves are called "user1", "user2" or "user3", depending on which save slot you want to decrypt. Encrypted files are 0x13E00 bytes long, decrypted ones will be 0x13DF8 bytes long.
Just use QuickBMS + decryption script + userX file to make it plain. With QuickBMS + encryption script + userX file, you can then re-import it onto your 3DS!
Here's pastebin links to the two scripts, i'll post them in full in the following posts for better readability.