S.H.
Well I have a 4 and 8 year old.
Girl and boy. The boy is the younger one.
My kids are very close. And adore each other. Play with each other and they like each other too.
However, I teach them that IF and when they need time alone or want to play by themselves, this is fine. They just have to say so, nicely. To me or the other sibling.
I teach them, everyone needs alone time, sometimes.
They understand.
My daughter has play dates. HER play dates with her friends.
My son has play dates, with HIS friends.
I do not combine them all the time, unless it is at my house.
Because I do often, have play-dates at my home for both my daughter and son, at the same time. A boat load of kids, at our house.
But with being invited to play-dates, I don't assume, the other sibling can go to the other's play-date either.
I tell my kids, they EACH have their own friends.
They understand.
I tell my son, "Sister has a play date today. It is our day today. What do you wanna do?" And visa versa.
When my son has a play-date... my daughter is actually glad. Because then, SHE has time to herself and it is QUIET. She relishes that. Although she loves her brother.
My son has a couple of times said if he can go with his sister on the play-date. I say no. Only sister was invited, and I cannot just invite ourselves. That is not polite. He has his own play-dates too. Just not today. He understands.
I always, explain to my kids. I don't combine them all on all activities. They have their own lives, too and interests.
Also, yes, a younger sibling will often tag along with an older sibling. I did. But, there needs to be, their own time too.
A 4 year old and a 7 or 8 year old, are VERY very different, developmentally. What a 7 or 8 year old does, is not the same as a 4 year old. Developmentally.
So.... you need to gauge, what you want your 4 year old to "mimic" in terms of what your 7 year old, may be doing, at that age. Because, it is drastically different, developmentally.