I can see your point of view, OP. A 4YO has a lot of big toys and having a separate playroom is better for everyone’s quality of life - no constant clutter and mess in the living areas, and she can leave out dens, railway tracks, Lego and craft projects for the next day without needing to tidy away mid game.
My DC all had en suites from birth, I’m not sure it is any more dangerous than a family bathroom. A child can just as easily get into trouble unseen in any bathroom.
My DC were pretty sensible but the 10-13 years were a bit of a nightmare with some of DS’s friends. They needed more supervision than toddlers 😂 Climbing scaffolding, skiing down the stairs, throwing water bombs, pushing each other in the pool fully clothed… (that was all just one friend at one party!).
I also agree with PP’s though. Four is too young to be sleeping on a different floor, you don’t want a playroom 2 floors away from the living areas, your DSS is getting to an age when he will need privacy more than your DD. It’s also not very practical to have the only bath in anyone’s bedroom, especially one who goes to bed at 7pm.
Do you have an alternative room that could be a playroom? Do you need the extra bedroom as an office or regular guest room? How often is your DSS there?
Without knowing the answer to those questions, I would suggest it’s better to have them both on the same floor as you. When they are older, they can have the top floor rooms for their later teen years. For now, make the top floor a shared kid’s space. Use the top bedroom as a guest room with a sofa or sofa bed so it can be a teen chill out/gaming space too. Use the seating area as toy storage and occasional playroom eg for play dates or games that need space. Or vice versa. It also means everyone has access to the bath.