Here's my thoughts, which hopefully will make you want to cry less rather than more.
1. The time around 4 is the worst of all possible ages for kids with these kinds of challenges. It is bad because fours tend to be more challenging as a general rule (the subtitle of the 4 year old book is "wild and wonderful" or something like that). Four is old enough you start to think they should know better. This is especially easy for bright ones as it is far too easy to expect they should know better. Four is an age when it is so easy to overestimate kids. Four is when you start looking toward school and worrying about the future. I know nothing about that may be helpful, but if I had a magic power to make moms not worry four be the age I'd choose to use it.
2. In your post I heard something that I'm very familiar with because I've been guilty of it too. You are all in one moment trying to deal with the problems of the past, present, and future. I get it, but I'm telling you it is a really destructive habit because it is way too much to worry about. It sounds like you are adding on problems and sadness of the future - he won't make friends, he will be big and violent, etc. And, you are also keeping ALL of the present problems and multiplying them exponentially. I get as a fear thing where that's coming from, but I don't think it is accurate or fair. Tons of four year olds who hit would never in a million years hit anyone by age ten. The fact that he feels comforted by only Pippi at age four says nothing about his tastes at age ten. If you think about the growth he's made from birth to the present time he's done amazing things. Learning to walk, talk, understand all sorts of things. He is radically different from the day he was born and four years from now he will be radically different than he is today. That doesn't mean of course he will grow and develop exactly like everyone else or like the most typical kid. He will probably continue to have asychronous, spiky development and he will grow and change in fits and spurts. But, the worst of today will ultimate seem like a distant memory.
3. One of the best techniques suggested to me to cope with some of this stuff is write it on the calendar and give yourself permission not to worry about it for a bit. If you are worried about a specific thing pick a time in the future - in six months or in a year. Is it still happening? If so, try to make a plan to deal with it. If not, let it go.
4. Try to pick individual problems and work on them a little bit at a time. Everything at once is too much. I would try to focus first on any problem that is really making him unhappy or hurting you. Tell yourself the other stuff is not something you have to deal with right now. I would look to anything that makes him feel happy or comforted as a positive. Even though it is understandable that you are ready to be done with Pippi if it brings him comfort it is a good thing. While it may not be typical to only want one story like that, I also won't say it is totally unheard of it developmentally typical kids either. There is a pretty big range of normal.