Have you read the thread below this one? Called 'All those horror stories, but what is the truth?' or very similar. We've got a bit sidetracked at the end, but Aeschylus asked a very similar question to this. I think it would be really worth a read, especially the first page
Your fears are completely normal. I think everyone probably has them, and they have your hopes as well
From my POV, two of my children have challenges. One is a mostly happy adult who has some issues, the other a very difficult teenager. I know I am very positive about adoption and what it has meant for them - but at least some of their issues will be life long. I guess whilst it has been unbearably tough at times, we have come through together as a family. In general I view adoption as something very challenging but also positive.
But, it is a leap of faith! A rollercoaster, full of unknowns, which you have to accept. I can't tell you what would happen if you adopted. Problems bonding are common (though not usually IME to do with biological connection. More to do with the time you have with your BC in your womb when you are bonding. You don't get that with an AC. They come fully formed with personalities before you have bonded at all). Not everyone has issues with bonding but it's common. I think it's a fear you just have to accept you have and make that leap. There's nothing else you can do really. No one can predict what will happen
Under 4's are largely unknowns I guess as well. No one can predict with much accuracy how they will fare as they grow up, or what effect their trauma will have on them. There are more uncertainties. Not every has lifelong difficulties, but some do. I think one of Mary's posts on the other thread addressed this best..couldn't add to it really. At some point you have to just say, well, you can hope for the best as everyone does, but at the point of matching you have to commit to work with that behavior as well as you can should it arise.
I'm a single now, and have been for a long time, but I know that my relationships got tested by all manner of things, some unexpected, and again, your fear is normal. But I can't say how you will react together
Only you can decide whether adoption is right for you, so I can't recommend as in say 'do it!' Only I would say it is something which brings out your fears, and has many uncertainties, and is a leap of faith..things you have to work through in your head at several points, this being one of them
I fear my answer might not be much help, I typed as I thought, and it's not any more specific than the question!