Your 4yo son sounds like my 3yo daughter. My daughter is constantly correcting what we say and/or how to say it. It's kind of a funny game. I'll say something (e.g., donkey). She'll say, "No, it's donkey" with a different inflection. etc. etc. I joke that it's like I'm trying to speak French and just keep getting the very subtle nuances wrong.
And another similarity. For a few months, if my daughter got her shirt wet (even a little), she'd insist on changing it. This was particularly unfortunate because she really loves to play around with water in the sink. She's getting better about this issue, but she still has her moments.
I suggest you look up the terms "spirited" or "strong-willed" child and see if you can find examples of what actions/behaviors these types of kids typically exhibit. I just got a book called "Setting Limits for your Strong-Willed Child," which I'm finding is a huge help. The best thing is it gives examples of some of the things spirited children might do. It may just be that you have one compliant child and one spirited one. I think of spirited kids as being really ornery, but really smart and independent (at least mine is!).
Anyway, I think it is highly unlikely that it's a discipline issue. If it were, both of your boys would probably be this way.
Good luck!