She’s 4 years old and every time I make her lunch, she comes home saying that she shared most of it with her friends. It’s not that I don’t like her being generous, but I’m worried that she’s not getting enough. I put fruit, a sandwich, yogurt, and some nuts in her lunchbox, but she only eats a little and shares the rest. What if she lacks energy? What if she gets hungry in class? I don’t want to forbid her, but I also don’t want her to go through the day without eating well
I can totally relate to this! My daughter does the same thing, and it stresses me out because I know she’s not eating enough. 
This was a huge concern for me too! I ended up explaining to my daughter that while it’s really nice to share, it’s important to keep enough food for herself so she can stay strong and healthy. I made a rule that she could share only if she had enough for herself first. I also tried packing slightly more food than usual and told her it’s okay if she doesn’t finish everything, but she needs to eat more before giving anything away. It helped her understand the balance. Maybe try talking to her about it and setting some boundaries while still encouraging the kindness? It worked wonders here! 
I think it’s great that she wants to share! My older son used to do the same thing at that age, and it made me nervous too. I started using a visual guide for his lunch, he had share zones a small section of the lunchbox for sharing, and the rest was for him to eat first. It sounds odd, but it gave him a sense of this part is mine, but if I still have room, I can share. I also tried offering more protein-based snacks, like boiled eggs or cheese sticks, which helped him feel fuller longer. It’s all about striking a balance and explaining the importance of taking care of yourself first. It worked for us, and he’s definitely not as generous with his snacks now that he’s older, lol! 
This happened with my daughter, and I had to set some ground rules for lunch, sharing. We talked about how it’s kind to share, but it’s also important to take care of yourself. We created a family lunch rule that included a no sharing unless you’ve eaten enough first policy. I also started putting in more filling snacks that didn’t tempt her to share as easily, things like hard-boiled eggs, apple slices with peanut butter, or granola bars. I would also make her lunch a little more visually appealing so it seemed like something she wanted to eat, like cutting the sandwich into fun shapes or making fruit salad. You’d be surprised how a little change can help!
You’re definitely not wrong for being concerned! I went through this with my daughter at the same age. She was always giving away so much of her lunch, and I was worried she wasn’t getting enough energy for the afternoon. Eventually, I started explaining to her how important it is to eat to keep her energy up for playing and learning. We also put a note in her lunchbox that said, Eat first, share second, which helped her remember. Over time, I found that packing a little extra food helped, she’s not likely to share a snack if she’s not hungry! It’s one of those balancing acts, being kind but making sure they’re eating enough. I hope you find what works for you! 
Ah, the sharing everything phase, been there! I ended up with an entire lunchbox of here, have some of my sandwich every day. 
We had to have a sit-down where I explained that her friends would survive without her snack lol and that sharing is awesome, but she still needs to make sure she eats enough to get through the day. She was really upset about it at first, but now, she gets it. It’s a tricky thing to navigate. Good luck, mama! You’re not alone in this