She needs to know... that SHE is okay...that SHE counts.... and validate her... even her icky feelings. Even adults need that.
Next, I had a SUPER overbearing/cantankerous/demanding sibling. I HATED growing up with her... I was the more mellow one, so I got the brunt end of everything... and everyone, because of my mellowness and ease of getting along... EXPECTED me to be 'able' to put up with my sibling.
NO. This is wrong to expect of any child, no matter how old or young. You see... being the door-mat of a sibling like this is MISERABLE for the other sibling... and as I got older, it caused a lot of stress and real emotional problems... because of the stress my BOSSY demanding sibling caused. You CANNOT CANNOT CANNOT let a child, "rule the roost." He will destroy the well-being of your other child. I know, I lived it.
No wonder, your girl is acting out. You need to realize.... she is very young... and behavior like this, is because of STRESS and inability to cope.... with her demanding brother who rules the roost. In other words... HE (whether for good or bad), gets his way and controls everything. That includes your 4 year old girl. You cannot expect her... to handle him, nor HIS problem.... like an adult. Adults even have a hard time handling it... as you said, he rules the roost. AND... he is creating a real problem, in your girl. Emotionally.
It doesn't matter how much one-on-one time you spend with her after he goes to bed... BECAUSE the whole DYNAMIC of the siblings and the family is affected. Your girl acting out and being "controlling" as you call it.... is a child's way of trying to COPE with the problem. When a child gets "controlling" like that in relation to your son.... she is merely trying to cope.
It is the dynamic that is in the family and that your son created.... created... that is affecting your daughter. ADversely.
As I said, I grew up with a sibling like that... and it was MISERABLE. It is not fair... nor age-appropriate for her to be expected to "know" how to cope.... with it.
You say she is going about it the wrong way.... but really, she is just being a 4 year old... who is not happy with the situation. And yes, her trying to have her own way... is really a symptom of her stress. And I personally, sympathize with her. A 4 year old... is not going to understand "how" to rationalize with or toward a toddler who is so demanding and ruling the roost and controlling your daily lives. That is even too much to ask of an Adult. No wonder she has a total melt-down when you try to address it with her... to a little child's mind, its still not fair and MORE than she can handle. Keep expectations of her, age appropriate and in refection of the havoc she must feel inside, because of her brother. Pick your battles... and simply let her be... too.
All the best,
Susan