Rooms based on need, not age.
I had the box room as a teen, because my DBs - blended family - shared. My best friend had the box room because her DSis has complex needs and needed more space for everything she needed. Both of us are eldest, but didn't feel "entitled" to the biggest room by virtue of that. Indeed, the middle room my brothers shared wasn't much bigger than the box room in some houses.
I didn't have a double bed until my second year of university. A lot of people I know continued to have singles through choice until long after they graduated. So a bit
at an 11 year old inherently preferring a double.
In this situation, DSD may realise she's ultimately happier with the box room, as it will be her own space. If a highsleeper would make the room feel too small, maybe consider a cabin bed with good drawers for storage. This would mean a small desk could fit without need for a wardrobe/drawers separately. If she wants to hang clothes, a wardrobe on the landing/in a sibling room, or a small hanging rail (less footprint than a wardrobe) if space allows.
The main thing is to reassure her that it's not because she means less - explain that the younger girls will be sharing because they both will still want all the little kid junk. They may want decor that she feels too old for - princesses, CBeebies, etc, while her own room can be exactly how she wants it.
Harder to explain why DS gets the big room and not her without some feelings of upset, but if it's his room she has a wardrobe in, for instance, she might feel less put out, as he's also compromising.
Four bedrooms for two adults and four children is plenty of space. There's just ways that not everybody will have exactly what they want - but they will all have what they NEED.