Whether or not you decide to have another child, your relationship will change and grow with your first. It will move to different levels and they will assert their independence regardless, kwim?
We have four children. I have treasured each and every stage even more with each succeeding child, and watching them grow together as siblings has strengthened my relationship with each child. They are so different from eachother, and their relationships with eachother are different, too. I feel that I have given them playmates and friends and confidants by giving them siblings to grow up with.
The decision you have to make is what kind of Family you want to raise and nurture and live with, not just how the birth of each child will affect the other(s).
I had my second when my first was 3 1/2, too. Ds1 started preschool when he was four and it helped me have some alone time with ds2, as well as letting ds1 begin to form friendships with kids his own age. Like I said, they have different needs at different ages and stages of devlopment, and he was ready for this step, not all 4yo kids are.
I don't mourn the loss of my relationship with ds1 anymore than I mourn the loss of my relationship with dh when ds1 was born. Things just changed, they didn't end.
Love and strength to you in making this important decision, I hope my experience helps you decide one way or the other.