Well...I think it would have been okay if he'd just said "Okay" again and left off the "I said" part, but then again, he's only four. I wouldn't have been offended if a four-year-old had responded to me in that way. An eight-year-old on the other hand, yeah, I'd probably be a little annoyed. I remember when I was three or four and talking to my grandfather on the phone, and responding to a repeated question with an annoyed, "Yeah, I already TOLD you!" My mother apologized to him, gently explained my mistake, and I apologized on my own. Mostly because I thought I would be punished, but I had meant no offense.
Maybe she overreacted, but sometimes adults seem to forget that kids don't think the same way, and certain things just don't occur to little minds.