I saw your post and it reminded me of my ds's request yesterday: "I want to go downstairs and I need a grown-up friend to come with me."

My 4 yo ds is the oldest, and he has a 3 yo sister with whom he plays a LOT. I think he does this by default since I am not a mom who typically "plays with" my kids. I provide tons of open-ended stuff, etc., and it's up to them to decide what to do. I do not get down on the floor and pretend to be the princess, etc. I think that is
their job, as kids! I do stick close by lots of the time so I can guide if needed when I hear inappropriate play coming on ("Let's pretend the bad guy is climbing on the window and swings from the blinds!" ...nope! let's not!), but really, it's their job.
If my ds needs something from me - art supplies, something he needs to pretend, etc., then great. I'll be more than happy to come over and see what he has built, etc., and I would love to help build the Lego castle one more time, or fix the Playmobil fire department.
However, when ds whines that he neeeeeeeeeds me to go downstairs with him while he plays, I usually gently remind him that I am (folding laundry/cleaning the kitchen/doing whatever) and if he wants to be with me, then I am right here and he can play nearby, if he wants. Otherwise, he is welcome to play downstairs by himself. He gets it, now, after having been through that a few times. Four is old enough to understand "No, I can't do that right now, but I would love to have you bring your stuff here in the kitchen so you can be with me while I work".

There are wonderful things about age four, and I am finding that this concrete understanding of everything is really nice right now. When you say, "No, I can't do XYZ, but YOU CAN do abc....", then I feel like they really understand that, whereas it was harder at ages 2-3.
