Hi K.,
My three daughters are now 20, 16, and about to turn 11. So the second was born just as the first was turning 4, and the third was born when the first and second were 10 and 6, respectively. Here's what I have learned from watching them grow...
As I wanted and expected, each child got full on "mama time" as she came into the world. SHE was the only baby of the family, with no older sibling still needing nursing, diapering, teaching to walk, talk, or even still home all day. So each one of them got ALL the babying she needed with NONE of the sibling rivalry more typical of children born closer together.
Each of the older siblings, as the new baby came into the home, were old enough to be on to the next phase of their lives--and got full on "mama time" in THAT phase, without the competition from a sibling in THAT phase either. Each of them was old enough to not only learn to love the new baby, but to really help--both at the birth itself, and with the new baby, who each time had a loving big sister or two who absolutely loved "showing her the ropes".
Each of the younger ones, as they became more able to, became the "mascot", "dress up doll", and "fairy princess" for the older siblings' friends to play with too, without too much difficulty when the older kids wanted to just go off and do their own thing, since there wasn't a desire on the part of the younger to be doing the same things the older kids did.
As my eldest has moved into adulthood, I would say the one downside has been that the youngest really doesn't have the strong connection with her, right now. She has a very close connection with the middle one--who has a very close connection with the oldest one.
As an older sister who was 11 when my youngest brother was born, and only 2 1/2 when my first brother was born, I have to say I have a MUCH closer relationship now in adulthood with my "baby" brother....mainly because we never had the kind of competition for parental attention that my middle brother and I had that STILL has impacts on our adult relationship.
So I have confidence that eventually my own eldest and my youngest may still have a close and strong relationship, once they are BOTH adults.
I think you will find that your oldest child will be very able to not only handle watching you manage the demands of a new baby, but will be VERY able to help out with your baby in ways that will amaze and delight all of you!
Blessings,
Fiora