Hey Heidi! dd is 4 too.
I have some thoughts on the whole thing for what it's worth: I have two elder sisters (5 and 7 years older, my mum had 1 mc between us). I didn't get on with either of them when they lived at home and our relationships have all ebbed and flowed between quite antagonistic and quite close over the years but we are now (mainly) on very good terms, as we have bonded over becoming parents. In fact one of my sisters was an absolute rock when I was struggling to come to terms with being a new parent. We are all very different people, but I know that I could rely on them in a crisis and vice versa.
Children are children for a very short part of their lives and their personalities will be in large part pre-determined (I believe), so they may or may not get along regardless of the age gap that's between them. All we can do is do our best job as parents to raise them to be well-rounded people and even if they don't appreciate one another now, maybe they will come to do so as adults and that will last for much longer.
Anyhow, I didn't plan to have such a big gap either but I am trying and failing to look on the positive side: when the baby comes dd1 will be at school and I will have the luxury of being able to give them my undivided attention for part of the time. It's not as if I have a choice anyway - if you had told me in my twenties that I would be this driven to have a child I would have laughed at you, but now I have an urge I can't ignore.
Suspect AF on it's way too, but trying not to lose hope yet...