S.H.
Have a child because you want a 4th, not because of how you "think" they will all grow up or bond or not having play-mate.
You cannot predict that, nor the gender of a child.
And, if you are concerned the Youngest child will NOT have a play-mate/sibling to play with... then where does that end? Because there will ALWAYS be a "youngest child" in the family.
So do you keep having another baby so the youngest is not the youngest anymore and so that he/she will not be "alone?"
Me and my sister are only 2 years apart.. that did NOT make it us being best friends, or closer, nor easier. On the other hand, my own kids are 4 years apart and they are VERY close and VERY bonded to each other.
It all varies by personality, how close or not, the siblings are... and if they will get along.
All the best,
Susan