I've heard many times that Sega's localization of Armored Core 4 obscures or remove entirely many details from the plot. However, the claims are rather vague, and I've seen at least one person state the claim is exagerated and the only thing the english translation obscures is that Fiona knows Joshua O'Brien personally. Not being fluent in Japanese, I can't check any of that for myself.
Would any Japanese speaker be able to detail what was changed? Or if it's just a fan myth?
While I don't speak Japanese, I can say that there were a number of lines that were somewhat modified from the original version of the game. For example, when Amazigh is defeated in the English version, he says ' This is the end...or are you?...'. The full line in the japanese version is 'So this is the end...or are you..for the same sake as...?'. That being said, there were also a number of unused audio files from the japanese version that weren't translated in the english version, as they had been dummied out of the original. This vid is a fan-made MAD that uses the japanese dialogue, including the cut content. You can see for yourself what was changed; it really wasn't as bad as everyone says. As far as storyline elements go, the only thing that was different, to my knowledge, was that Joshua was supposedly identified as the 00-ARETHA's pilot at the end of the japanese version in some way or form, whereas the english version leaves it somewhat unclear unless you're paying close attention to the voice actor.
Wow, thanks for the link! I didn't expect a response so soon.
After watching that video, it does seem the English translation is close to the Japanese more often than it's not, to the point I was able to recall the english equivalent instantly. The only real differences I noticed beside the one you pointed out is Fiona saying "You and Joshua are alike, but I think you're even greater" (which I suppose is from the debriefing of Sol Dios - the English translation leaves out the "You're even greater" part) and Joshua saying "Hey Fiona! It's been a while!" (though that could be one of the dummied lines you mentioned?).
Now that I think of it, I do recall Fiona's dialogue at the end of Juggernaut (which isn't showcased in that MAD) being much longer in Japanese, but I can't be arsed to find it atm.
theres also this amv
https://www.youtube.com/watch?v=G-7aRd0NPPU
one example is the part around 00:35