A 4yo is not all that much older than a 3yo when it comes to sharing. I suggest that you accept that you did the best this time and plan to be more observant and proactive next year if you see that your daughter is expected to be different than the one who is just one year younger. These kids are in the same age group when it comes to an issue like sharing. They are close enough in age, it's best to have the same rules for all of them. A preschooler cannot understand the difference in expectations. Actually, when I think of sharing, I can't understand the difference.
Forgive yourself for falling into the "how great my kid is," trap. You've learned and you'll do much better next time.
Consider the possibility that the other mother knew her kids weren't good at sharing, which, by the way is normal, and so didn't expect them to share because she was doing the same thing, only in reverse, as you were. She may have not wanted for you to see that her kids couldn't share. She didn't want to deal with the temper tantrum if your daughter didn't share. She wanted to look like the "good" parent and wasn't thinking about what your daughter needed.
I suggest that at the beginning of next years trip you have some ready answers regarding your daughter in possible situations. The kids will be older. Think about your daughter and her developmental stage so that you can stand up for her. The key is knowing what is reasonable to expect from your daughter and advocate for her.
Perhaps be prepared to talk with your friend about what you're doing if it appears that your standing up for your daughter is causing tension. Talk about developmental stages and how the children are close enough in age that they have similar needs. Be careful to come across as concerned about all of the children and non-judgmental. Focus on helping all of the children feel comfortable while acknowledging that there has to be a give and take amongst all of the children.
Remember, this was a learning experience for you. You hadn't had this experience before. Also you were with good friends with whom your relationship is involving because of the addition of children. You did the best you could. Now, you've learned and are looking for a better way. Please do not be hard on yourself. As you said, no harm was done. Everyone are still friends and you will all grow together.