it sounds as if you are so scared that you will next want a fifth, then a sixth, any number to keep those babies around you, but they will grow up, and then whichever number, you will have an empty nest, and you will at that stage have to rely on yourself to get through life, and if you have no other interests, what then? You could be very sad and desolate.
A person with a lot of outside interests is a very interesting person in herself, and lives a full life.
A child is a lot more expensive than a hobby or studying.
You are a lot more than just a mother. Surely you don't want to just live through your children, particularly restricting this to babies - there's a whole wide exciting/manic/upsetting/huge important world out there - there's politics, wars to be interested in, geography/ nature/ fashion/ whatever floats your boat. Surely you'll be a better parent if you are informed about the world, actively setting an example to your children.
Why not volunteer at a nursery/train to be a child minder/a part time nanny, something to perhaps 'grow' you and still assuage this craving.
But also - perhaps get a few more non-child-related hobbies and interests for yourself, or, something like playing an instrument or learning a sport which you could do with your children and your dh to enrich their lives.
Don't just to live through your children - you don't want to be someone who can only talk about babies, boring, boring, boring, (to some of us anyway) .
Why not volunteer for a children's charity- whether an international one or a local one. Something like a charity for child soldiers, or starving babies, there are loads of children's charities, there must be one which interests you. Do something outside of the family to make your children and your dh proud of you. You have so much potential to do a lot of good - it would be a great example for your dcs, and it would help you too.
Just be brave and put your toe in the water.