OP I mean this very kindly but I think you are over thinking this hugely.
He will be absolutely fine, my experience is that children grow and blossom in a whole new way once they attend preschool.
When he asks about why his brother doesn't go, then you just say 'you are a big boy and this is what you do now - how exciting! Of course X (little brother) can't go too, he is still a baby but he will go when he is big like you are now'.
When you say that you will pull him out if he doesn't settle, think about what that will mean. He will then have to start school having had no experience of that type of setting - and you can't pull him out of school because he will be 5 so soon after he starts. Unless of course you would plan to home ed him.
You really need to get a bit of a grip here, for his benefit, and try and focus on the positives. He gets to learn to be a little independent from you, which is a good thing in readiness for school, and you get to have some one on one time with your youngest and develop your relationship with him.
It will be fine, honestly :)