This is probably something you know if you read my thread on circumcision, but the only thing I've hated about my body is the fact that I'm circumcised. I have buckteeth and I'm more focused on my lack of a foreskin than a space in between my teeth.
I mean, honestly, I've just always preferred the aesthetics of intact genitals because to me, the circumcised penis looks scarred and the head looks all dried up, so honestly I'm kind of in a situation where I had a cosmetic surgery performed on me without my consent, which ended up also going against my preferences regarding aesthetics. My feelings about it only got worse when I started researching the foreskin's functions, so that only made me feel even more like something was stolen from me. That sealed the deal. I was going to do something about it, no matter how extreme the measure was.
So, to answer question 4, yes, I am currently undergoing a body modification. I'm currently taking measures to restore my foreskin - well, sort of a foreskin. The idea behind this is that it works on the principle of tissue expansion - that is, if you stretch the skin, it encourages cell division, so I'm simply applying a method like this to stretch skin over the head of my penis. This will give me back the foreskin's protective functions, as well as bringing my body in line with my own aesthetic preferences, but any sensation in the foreskin that I would have had I can't get back. Honestly, to answer how I feel about it, it gives me hope. It makes me stop feeling like I'm trapped with a body part I hate the appearance of (and that I consider defective) when I never even had a say in whether or not I wanted it modified, so it resolves my insecurities over my penis and makes me feel whole again. I've still got a long ways to go (hey, stretching out 15 square inches of skin takes a while) but it gives me hope.
To answer the other questions:
1 - My response to this thread probably answers how I feel about body modification. If you don't like your appearance, do something about it. Sitting around and groaning about it isn't gonna help.
2 - There are probably scenarios in which correcting your physical image is preferable over correcting your self perception and vice-versa. I'm not gonna lie, my feelings about having had my foreskin chopped off are far from usual, but to be frank, I spent the entire fall quarter at my college going to therapy sessions every week and they didn't help at all, so I kind of don't have much of a choice but to correct my appearance to something I can accept rather than futilely trying to live with an appearance I can't accept. However, there are also cases where negative body image can be caused by society setting impossible standards (models, bodybuilders, etc.) so if you're trying to reach a standard that's impossible to reach without like steroids or something, then you just need to sit down and think about how you perceive yourself.
3 - In principle? No. The difference here is degree. As already stated by others here, there's a big difference between getting your teeth whitened and getting a nose job, but the differences there are in degree.
That just about settles my response to the OP