OP I could have written your post two years ago. In fact, I had to check the name and date to make sure it wasn't one that I had forgotten writing!
I had a MMC at 12 weeks after trying for a year, when DS was just coming up to 3. I had that panicky feeling that getting pg was completely out of my control and that poor DS was getting older and older without a sibling. I didn't know when or if I would be able to have another child and the not knowing made me even more panicky.
I didn't get pregnant again for another year and then had another MC (I have pcos which may explain our fertility issues). All that time I worried and panicked about the age gap. But DS was blissfully unaware! His life went on as normal, he went to nursery, made friends, had a baby cousin to pour love and affection on. He is the apple of his grandparents eye and he got full attention from us. When he started school in September, I was able to give him my full attention.
Just as he started school, I found out I was pg and this one stuck! I am due on Saturday so fingers crossed all will be well.
It turns out that there are lots of reasons why getting pregnant when you have a four year old is actually pretty good! I don't find being pregnant easy and so it has been a godsend having DS at school because I get to have two days to myself (I work PT). This last few weeks on mat leave I have been pottering about enjoying my last few days of freedom like when I had DS. DS is fully aware and excited about having a baby brother or sister. He loves telling people he is going to be a big brother and I am less worried about the impact on him than if he was a toddler. I have friends with kids that have smaller age gaps and although they wouldn't change it for the world, they tell me that it ain't easy having two little ones! My big nearly five year old is now able to dress himself, entertain himself, get his own shoes on, swim on his own, ride a bike.... All things that will keep him busy whilst I grapple with a newborn!
What I am trying to demonstrate is that although the age gap issue may seem like a massive deal at the moment, it hopefully won't always. I think personally it was something that I fixated on unnecessarily -is it called projecting or something? I worried about that when really I was worrying about the wider issue of the miscarriages and not being able to get pregnant.
I am really happy that things have turned out as they have for us so far, I can see the positives now in a bigger gap and am grateful that the baby we have coming now is here at all.
There is a six year age gap between me and my sister and although we didn't exactly see eye to eye all the time when growing up, (which siblings do?) we always had a very strong sibling bond and are now absolutely the best of friends. As people so often say, it's the personality that counts and not the age gap.
Good luck with trying again when you're ready, I really hope that in a few months time you are able to write a similar post to mine telling someone else that in all probability things will turn out fine!