My two kids have the same age difference but are now 5 and 3. Neither of them are in preschool programs so they are together all day, every day. In our family, we have a good perspective on this because I am the youngest child in my family and my husband is the oldest.
We understand that while the older child can sometimes "torment" the younger one, we want to guard against our younger child from feeling like a "victim" and using that position to get the older one in trouble. My husband says he felt like he pretty much always got the blame when something went down, just because he was the oldest and "should know better."
One thing we've done is (this might sound strange but stay with me here) is encourage the kids to work together, even if it's against us. We want to encourage a relationship between them where they almost feel like it's "them" (the kids) against "us" (the parents). The other thing is we don't allow any tattling (unless it's safety issue like swallowing something or fingers in outlets). So "She hit me!" and "He took my toy!" don't have any power. Obviously this applies more to when they get a little older.
For now, I would not make it about who is bigger or smaller, older or younger. I would make it about the rules. If he hits her, remove him from the situation (the corner, a chair, his room, etc) with a firm "We don't hit in this house." After you give your youngest, yourself, and your oldest a few minutes to calm down, calmly repeat "We don't hit in this house" and move on. Our rule for toys in our house is "whoever is playing with it has possesion" so we don't get bogged down in whose toy belongs to whom. That has helped because all that matters is the rule, which is "We don't take toys away from other people." This eliminates the "that's miiiiiiine" whines.
For the time being, your youngest needs you to be her advocate since she probably can't speak for herself, but soon enough she'll be defending her space on her own! Good luck.