In the actual BH4 script written by Mikami, Leon's personality is much closer to his BH2 personality. He is serious and mission-orientated, cracking very few jokes and generally not talking very much at all. His Smarmy McCockface personality in the RE4 localization was added by the translator, Shinsaku Ohara. Who, like Mikami himself, is a pretty awful, awful writer. The reason Leon is more serious in Degeneration and beyond is because Shinsaku Ohara no longer works on translating the series (thankfully) and other translators kept Leon's personality intact (while fucking up other things). For example, when Hunnigan says she was worried about him after several hours without contact, Leon replies "Don't you mean lonely?" in the English script. In Japanese, he simply says: "...Sorry." His "Monsters. Guess after this there'll be one less to worry about." line in English is simply "Another monster?" in Japanese. I think the only line intact is "Your right hand comes off?".
Basically, if you understand Japanese and English and are playing the Japanese version of the game, the subtitles do not match up with the spoken English dialogue. This is true for every game in the series, but when a character's entire personality is completely different, something is fucked up.
Ada is also less... whatever she is in the Japanese script, and is much more withdrawn and mysterious. She spends little time actually talking to Leon, deflecting everything he says and ending every last one of their conversations with "See you.", even when Leon is mid-question. Closer to her BH2 personality. Expect her personality to be quite a bit different in BH6 compared to her portrayal in BH4.
Saddler also isn't some U.S. hating terrorist. In fact, there are no actual terrorism or even U.S. hating references. This was all added by Ohara, who I suppose wanted to try being a topical political commentator. For example, in the first meeting with Saddler, rather than "No longer will the United States think they can police the world forever", he actually says "The United States is just right for demonstrating my power to the world". The terrorism line with Salazar is not there in general.
BH4's localization is still one of the better ones in the series. However, this is only because Mikami's original scriptwriting was shit to begin with and there wasn't a lot of plot for the translator to fuck up. Just dialogue.