Most likely. Sorry. ._.
You see, in 4th gen the PID (which accounts for nature, shininess, gender, ability, stuff along those lines) and the IVs have a link when capturing so that only certain PIDs can be paired with certain IVs. In 5th gen this is not the case.
Currently, the only natures you would be able to get absolutely flawless in 4th gen are modest, timid, calm and docile. I think bold and sassy are possible too from using a pokemon with cute charm while RNGing, however I haven't really done it so I'm not entirely sure about that. But the 4 I know for sure are modest, calm, timid and docile.
If you are unsure about this, I can pull up the IV spreads from PokeRNG for concrete proof. I've RNG'd enough standing legendaries with these spreads before too, if that helps.
But please remember that this link only applies to captured pokemon in 4th gen games. Eggs and wondercards are generated differently.